I was searching for other calendar projects, but did not find any pages - but I believe that also others had the same thoughts before. On the projects proposal page the Almanac and the Wikihistory projects seem to be related.
Anyway: this is definitely a Wikidata project that takes advantage out of the WiktionaryZ ERD.
Working with rare languages contents creation often is a problem. The calendar is one of those things people can easily work on, but: each calendar has other dates, many of them would be interesting for all Wikipedias. Sorting and a proper structure is required. For now most projects take one wikipedia as example and translate the calendar pages - then they add their own dates step by step. This leads to calendars/almanacs that contain the same data (80-95%).
Now what would make sense is have a unique data repository for events, feasts, births, deaths etc.
Needed fields and options 
- description (with the possibility to insert wiki linking + html + pictures)
- checkbox for "inclusion" in the local calendar (it could be that certain events are not wanted to be shown on certain wikipedias).
- attribution to types of events - basics are: evnts, feasts, births, deaths etc. - this can then go deeper into "economical, political etc. event"
This wiki is a multilingual wiki, that means we will have translations of all events etc. The events show automatically up on the wikipedias once they are present in the specific language.