Maariv (and Kiddush) should preferably not begin before Tzeit Hakochovim (8:26pm) so that one can complete the full Sefirat Haomer cycle.
If someone davened Maariv before this time he should read Kriat Shema for Maariv before the Seudah.
After Maariv one should hasten to start his Seudat Chag as the day is short with plenty to do (to be busy in learning Torah all night).
It is good and correct not to overeat or drink as overeating or drinking will make one tired.
One must drink a cup of wine on the day of Chag, even by those who did not make their own Kiddush.
One should speed up their meal in order for them to go and learn.
According to the Ari Z”l, one should refrain from marital relations, unless it is the night of Mikvah.
Ya’aleh Veyavo in Bentching
If one forgets to recite Ya’aleh Veyavo in Birchat Hamazon, but remembers before Hashem’s name at Bonei Yerushalayim, he goes back. If one realized after that, but before starting the next Brachah, he recites the extra Brachah as printed in some Birkonim. If one already began even the first word of the next Brachah, one must begin Birchat Hamazon again.
The Harachamon for Chag is recited.
