Shulchan Aruch Orach chaim 494:3 - שולחן ערוך אורח חיים סימן תצד סעיף ג
אסור להתענות במוצאי חג השבועות: - It is forbidden to fast on the Motzai Shavuos.
Usually the term ---מוצאי Motzai--- refers to the night following, as in Motzai Shabbos or Motzai Yom-Tov, which means the night folowing the day of Shabbos or Yom-tov.
The Biur HaGra in explanation of this halacha writes:
כמ"ש בפ"ד מסוכה כל העשה איסור לחג וכו' וכפי' השני של רש"י..ובפ"ב דחגיגה מעשה ומת אלכסא בלוד ובאו כו' ולכן כ' כאן
This is based on the Gemara in the fourth perek of Sukkah (45a) which says
Anyone who establishes an addition [issur] to the Festival on the day after the Festival by eating and drinking, ... This is going according to the second explanation of Raashi (ibid) that Issur L'chag is referring to the day after the Festival.
The Gra then explains that because a certain story brought in the gemara in Chagiga (which is relevant to this subject) happened to take place on Isru chag of Shavuos, therefore this halacha, of not fasting on Isru-Chag, is brought in hilchos shavuos, although it pertains to any Yom-Tov as does the Gemara in Sukkah.
In Shulchan Aruch Hilchos Pesach 429 הלכות פסח סימן תכ"ט the Remah writes:
ונוהגים להרבות קצת באכילה ושתיה ביום אחר החג והוא אסרו חג
It is customary to somewhat add in eating and drinking on the day after holidays, And this is [called] Isru-chag.
The Biur HaGra there writes:
אבל מותר להתענות - But one is permitted to fast. - (And he proves it from a gemara in Taanis and in Chagiga).
This seems to say that it is customary to eat on Isru-chag but not mandatory.
This seems to contradict the Gra cited above which says that the halacha brought in the shulchan aruch that It is forbidden to fast on the Motzai Shavuos, is not specific to Shavuos, but pertains also to all holidays, and fasting on Isru-chag of any Yom Tov is forbidden.
