If salaried the vast majority of contracts will say in arrears, ie they pay fo what you have done, not what you will do.
If your monthly paid then the part month at the start will have been settled at some point, probably your first pay was 1.x months.
Most contracts also state "by the end of the (calendar) month", even if they in practice pay in advance of that date. Early payment gives them some breathing room should something go wrong, and it can. If they didnt pay by the end of the month they would be in breach of contract.
I get paid on the 15th, sometimes as early as the 13th, my contract says, monthly in arrears, by the end of the calendar month.