PTRH 57.b:49[2] :בגין מודעא דאיתקדש ירחא‏ : in order to inform them that the new moon had been sanctified
See the entire entry for bgyn conj. because, since

PTRH 57.b:49[2] :בגין מודעא דאיתקדש ירחא‏ : in order to inform them that the new moon had been sanctified
See the entire entry for yraḥ (īraḥ), yarḥā n.m. month