specific-holidays-gcal is for those who want to use Google Calendar's regional holiday calendars but only want to show specific holidays instead of all of the included holidays.
For example, GCal's "Holidays in United States" calendar includes many different holidays, but if you only want your calendar to show New Year's Day and Valentine's Day, then this makes that possible.
See this website to easily select specific holidays inside of any given Google Calendar regional holiday calendar .ics file. This is for the SPECIFIC_HOLIDAYS setting in the project above.
The Apps Script project is a modified version of GAS-ICS-Sync, created by Derek Antrican and other contributors.