Is the Android default calendar open source? I did a little of research and found nothing.
All Android git repos are here https://android.googlesource.com and the Calendar one is here: https://android.googlesource.com/platform/packages/apps/Calendar keep in mind that this is the stock AOSP calendar, not the Google Calendar however they function the same. Here is another open source calendar: https://github.com/Etar-Group/Etar-Calendar that is probably closer to Google's implementation rather than the AOSP one.