Is it sunat to fast on 12 rabi ul awal? When is it tmrw?
It’s Sunnat to fast on Monday and Thursday.
12 rabiul awwal this year (1434AH/2013AD), is on Thursday 24 January.
Fast if you can, and send the sawab of the fast to Rasulullah sallallahu alayhi wasallam. Also give some Sadaqah on his behalf, read some Quran for him too. And make special dua for the hidayat of his ummah.