This script lib/main.js to compute the liturgical celebrations of the day is self-contained with 0 dependencies, so it may be simply loaded in a <script> tag. It was written with the Archdiocese of San Francisco in mind but is suitable for any diocese with the same rules (eg. Epiphany and Ascension on Sunday).
All you need is a function call to getCelebrations(cache, y, m, d) where
cache is an object to which you want to hold a reference (so that you can call
getCelebrations again and it doesn't have to recompute the whole year). y, m, d
are the date components. Note that m (the month) is 0-indexed, like in JavaScript's
Date object, but d is 1-indexed.