The day after Pesah, Shavuot and Sukkot is known as Isru Hag. The Gemara (Sukkah 45b) expounds the verse that is sung in the Hallel “Isru Hag Ba’Avotim Ad Karnot HaMizbeah” to mean that one who treats the day after the holiday festively, is considered to have brought a sacrifice to the Bet HaMikdash. Furthermore, the Yerushalmi (Avoda Zara 1:5) refers to Isru Hag as “Bere DeMoada” (lit. “child of the holiday”) on which it was customary to eat a festive meal. The Rama (Orah Haim 429:2) states that one should treat this day as a semi-holiday by eating and drinking. Indeed, Rabbi Yosef Benaim (Noheg BeHochma, pg. 34) explains that the custom in the city of Fes was to refrain from work on Isru Hag. The Ben Ish Hai (Rav Pe’alim, vol. 2, Sod LeYesharim § 1) states that this was the custom in Baghdad as well, but only on the Isru Haf of Pesah and Sukkot, but not Shavuot. The prevalent custom nowadays is to work on Isru Hag. Furthermore there is a custom to recite the Mizmor “HaShamaim Mesaperim Kevod E-l” and the verses “Ana Hashem Hoshia Na, Ana Hashem Hatzliha Na”.
Summary: Isru Hag has a semi-festive character.