When:
May 12, 2021 all-day
2021-05-12T00:00:00+02:00
2021-05-13T00:00:00+02:00

Beginning of new Hebrew month of Sivan. Sivan is the 3rd month of the Hebrew year. Corresponds to May or June on the Gregorian calendar

https://www.hebcal.com/holidays/rosh-chodesh-sivan-2021?utm_source=js&utm_medium=icalendar