In OC Siman 695 Sif 3 that if one starts their Purim seudah by day and finishes by night they should still say "Al Hanisim" in Bircas HaMazon. The Machaber brings an opinion that says to not say it; ...
Is there a requirement to have a meal on the night of Meggilah reading? If not, why is there a custom to eat one? Is one required to drink even at night?