Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answer updated 5 months ago

It starts on the previous day at sunset. Example: 

---------------------------------------------------------
|     Saturday, February 13  = 1 Rajab        |
---------------------------------------------------------

1 Rajab starts on Friday at sunset and ends on Saturday at sunset.

Hope that helps!