"FullCalendar is a jQuery plugin that provides a full-sized, drag & drop calendar like the one below. It uses AJAX to fetch events on-the-fly for each month and is easily configured to use your own feed format"
Source: https://fullcalendar.io/

It started as a preference for the Calendar in Tiki8 and improved in Tiki9.

Later, a fork was used to get resource views as well for PluginTrackerCalendar

Later, FullCalendar added resources views an option and we reverted back the main project.

In Tiki22, FullCalendar becomes the official (only) interface and legacy HTML calendar code is removed.