Best Open Source JavaScript Calendar Components in 2026

Choosing an open source JavaScript calendar component in 2026 means sorting through a crowded landscape. FullCalendar still dominates the download charts, but several newer libraries have emerged that challenge the assumption that a full-featured calendar must ship 40+ KB of JavaScript. This guide compares the top contenders by bundle size, features, licensing, and real-world trade-offs — so you can pick the right one for your project.

The open source calendar landscape

Six libraries stand out in 2026. Each is open source, actively maintained, and works in production applications. But they differ sharply in architecture, weight, and what they include out of the box.

FullCalendar — the established default

FullCalendar is the most widely adopted JavaScript calendar with over 19,000 GitHub stars and roughly 1 million weekly npm downloads. It is the library most developers reach for first — and for good reason. The ecosystem is mature, the documentation is thorough, and the community is large enough that most questions have existing answers on Stack Overflow.

The trade-off is architecture. FullCalendar v6 adopted Preact as its internal rendering engine, which increased the core bundle size significantly. A minimal setup requires at least two packages:

npm install @fullcalendar/core @fullcalendar/daygrid

import { Calendar } from '@fullcalendar/core';
import dayGridPlugin from '@fullcalendar/daygrid';

const calendar = new Calendar(document.getElementById('calendar'), {
  plugins: [dayGridPlugin],
  initialView: 'dayGridMonth',
  events: [{ title: 'Standup', start: '2026-09-03' }],
});
calendar.render();

That is ~43 KB gzipped before adding drag-and-drop (@fullcalendar/interaction) or time-grid views. A full-featured setup with four plugins easily exceeds 60 KB gzipped.

Licensing

Standard plugins are MIT-licensed — free for any use. Premium plugins (resource timeline, resource views) require a commercial license starting at $599/year.

Event Calendar — resource views at lower weight

Event Calendar by Volodymyr Kurko has quietly built a strong following with 2,300+ GitHub stars and 43,000+ weekly npm downloads. It ships a full-sized drag-and-drop calendar with resource and timeline views — features that typically require FullCalendar's paid tier.

At ~35 KB (Brotli compressed) as a standalone bundle with zero dependencies, it sits between FullCalendar and lighter alternatives. The API is FullCalendar-compatible by design, making migration straightforward.

import Calendar from '@event-calendar/core';
import DayGrid from '@event-calendar/day-grid';
import TimeGrid from '@event-calendar/time-grid';
import Interaction from '@event-calendar/interaction';

const ec = new Calendar({
  target: document.getElementById('calendar'),
  props: {
    plugins: [DayGrid, TimeGrid, Interaction],
    options: {
      view: 'dayGridMonth',
      events: [{ title: 'Review', start: '2026-09-04', end: '2026-09-04' }],
    },
  },
});

The main trade-off: while fully open source under MIT, the community and documentation are smaller than FullCalendar's. You will rely more on reading source code and GitHub issues than browsing cookbook-style docs.

Schedule-X — the modern challenger

Schedule-X positions itself as a modern alternative to FullCalendar and React Big Calendar, with 2,600+ GitHub stars and framework adapters for React, Vue, Angular, Svelte, and Preact. It focuses on responsive design, internationalization, and extensibility.

The core is open source under MIT. Premium plugins (drag-and-drop from external sources, resource scheduling) are available under a paid license. The architecture is Preact-based internally, similar to FullCalendar v6.

import { createCalendar, viewMonthGrid } from '@schedule-x/calendar';
import '@schedule-x/theme-default/dist/index.css';

const calendar = createCalendar({
  views: [viewMonthGrid],
  events: [
    { id: '1', title: 'Sprint Review', start: '2026-09-05 10:00', end: '2026-09-05 11:00' },
  ],
});
calendar.render(document.getElementById('calendar'));

Schedule-X is the newest library on this list, which means the ecosystem is still maturing. If you need a battle-tested solution for a production app today, that youth is a risk factor. If you are starting fresh and value a modern API, it is worth evaluating.

Calendar.js — maximum features, MIT licensed

Calendar.js by William Troup takes the opposite approach to minimalism. It ships eight views (month, week, day, year, timeline, all-events, date picker, widget mode), 52 language translations, drag-and-drop, recurring events, and data export to CSV, JSON, XML, iCAL, and more — all in a single zero-dependency file under the MIT license.

const calendar = new calendarJs('calendar', {
  exportEventsEnabled: true,
  manualEditingEnabled: true,
});

calendar.addEvent({
  title: 'Design Review',
  from: new Date(2026, 8, 5, 14, 0),
  to: new Date(2026, 8, 5, 15, 0),
  isAllDay: false,
});

The trade-off is weight. Calendar.js ships at ~40 KB gzipped — comparable to FullCalendar's core. Because it is a monolithic single file, you cannot tree-shake features you do not use. If you need its full feature set, that weight is justified. If you only need month and week views, you are paying for year views, timeline views, and CSV export you will never call.

React Big Calendar — the React-only option

React Big Calendar has 8,500+ GitHub stars and close to 500,000 weekly npm downloads. If your entire application is React and will stay React, it is a natural fit — events, views, and interactions are all expressed as React components and props.

import { Calendar, momentLocalizer } from 'react-big-calendar';
import moment from 'moment';
import 'react-big-calendar/lib/css/react-big-calendar.css';

const localizer = momentLocalizer(moment);

function MyCalendar({ events }) {
  return (
    <Calendar
      localizer={localizer}
      events={events}
      startAccessor="start"
      endAccessor="end"
      style={{ height: 600 }}
    />
  );
}

The trade-off is lock-in. React Big Calendar only works inside React. It also requires a date localizer library — Moment.js (~70 KB gzipped), Day.js (~3 KB), or date-fns (~12 KB) — as a peer dependency. If you migrate to Vue, Svelte, or a server-rendered stack, the calendar cannot come with you.

SimpleCalendarJS — full calendar at date-picker weight

SimpleCalendarJS occupies a unique position in this landscape. It delivers a full interactive calendar — month, week, day, and list views with drag-and-drop, event resizing, and async event loading — in ~14 KB gzipped with zero dependencies.

npm install simple-calendar-js

import SimpleCalendarJs from 'simple-calendar-js';
import 'simple-calendar-js/simple-calendar-js.css';

const calendar = new SimpleCalendarJs('#calendar', {
  defaultView: 'month',
  fetchEvents: async (start, end) => {
    const res = await fetch(`/api/events?from=${start.toISOString()}&to=${end.toISOString()}`);
    return res.json();
  },
  onEventClick: (event) => console.log('Clicked:', event.title),
  onSlotClick: (date) => console.log('Slot:', date),
  enableDragAndDrop: true,
  enableResize: true,
});

One package. One CSS import. One constructor call. No plugins to coordinate. No peer dependencies to align.

What makes it different

• 14 KB gzipped — lighter than most date pickers, smaller than any other full event calendar
• Zero dependencies — works anywhere JavaScript runs
• Month, week, day, and list views included out of the box
• Async event loading via fetchEvents — no need to pre-load all events
• Drag-and-drop and resize without an additional interaction plugin
• CSS custom properties for theming — dark mode is a few variable overrides
• Official React, Vue 3, and Angular wrappers ship in the same package

Theming

.uc-calendar {
  --cal-primary: #2563eb;
  --cal-primary-dark: #1d4ed8;
  --cal-today-bg: #eff6ff;
  --cal-font-size: 14px;
}

Pricing

SimpleCalendarJS is free for personal and open-source projects. Commercial use requires a paid license — $49/year or $199 lifetime per project.

Choosing the right open source calendar

The decision comes down to three questions:

How much JavaScript can you afford to ship? If you are building a performance-sensitive application — a SaaS dashboard, a mobile-first PWA, a content site with Core Web Vitals targets — every KB counts. SimpleCalendarJS at 14 KB is three times lighter than the next full-featured option.

Do you need resource scheduling? If your use case involves assigning events to named resources (rooms, staff, equipment) on a timeline, Event Calendar and FullCalendar Premium are your realistic options. SimpleCalendarJS, Schedule-X, and React Big Calendar do not offer resource views.

Are you locked to a framework? React Big Calendar only works in React. Schedule-X is Preact-based internally. FullCalendar ships separate adapter packages per framework. SimpleCalendarJS, Event Calendar, and Calendar.js all work in vanilla JavaScript and inside any framework.

Summary

• The open source JavaScript calendar landscape in 2026 ranges from 14 KB to 60+ KB gzipped — a 4x difference that directly impacts load times and Core Web Vitals.
• FullCalendar remains the most adopted option with the largest community, but its plugin architecture and Preact dependency push the baseline to 43 KB+ before you add common features.
• Event Calendar offers resource and timeline views under the MIT license at ~35 KB — features that cost $599/year with FullCalendar Premium.
• React Big Calendar is a strong choice for React-only projects, but locks you to React and requires a separate date localizer library.
• SimpleCalendarJS delivers the smallest full-featured calendar at ~14 KB gzipped with zero dependencies, drag-and-drop, and framework wrappers — covering the needs of the vast majority of calendar use cases.

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Sources & Further Reading

Research & References

• FullCalendar — JavaScript Event Calendar
• FullCalendar Bundle Size Issue #7029 — GitHub
• Event Calendar (vkurko/calendar) — GitHub
• Schedule-X — Modern JavaScript Event Calendar
• Schedule-X — GitHub
• Calendar.js — JavaScript Calendar Components
• Calendar.js — GitHub
• React Big Calendar — GitHub
• The Best JavaScript Calendar Components — Bryntum Blog
• Top JavaScript Scheduler Libraries in 2026 — jQWidgets

Image Credits

• Cover: Turned-on Computer Monitor Displaying Text — Pexels

All images free to use under the Pexels License.