Q. We often have our children and their families staying with us for Pesach and leaving the day after traveling home. Is it worth it to celebrate with them before they leave a seuda or special meal on Yisru Chag? Would that be a Seudas Mitzva?
A. Shulchan Aruch (O.H. 429: 1) writes that it is tradition to add a bit to the eating and drinking on Yisru Chag. It is considered as if one built the Mizbeach and sacrificed on it all Korbonos.
There are different reasons for this particular tradition. Chassam Sofer (ibid) explains that it commemorates the joy of the people who came to Yerushalaim and now after complying and enjoying so much the Yom Tov, were ready to return home. Torah Lishma (140) quotes Arizal that the Kedusha and holiness of Yom Tov is still felt on that day.
He also quotes Yerushalmi (Avoda Zarah 1: 1) that Yisru Chag is called 'Briah Demoado' or the child of Yom Tov (See Nitei Gavriel 22: 4).
Horav Dovid Pam's opinion is that since after all Shulchan Aruch mentions only to add a bit to the food one eats, if the visiting children are pressed to return home, as often is, it may be better to avoid.
Horav Shlomo Miller’s Shlit’a opinion is hat one should follow the traditions of one's family.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch
Ehrentreu Shlit'a