…there is no Talmudic evidence at all in regard to any importance attached to Isru Hag. There is, in fact, evidence to the contrary. The Talmud states: “The days that are written in the Scroll of Fasts the days before and the days that follow them are (also) forbidden; but as for Sabbaths and festivals, on them (on the very day of the Sabbath and festival) it is forbidden but on the day before, and on the day following, it is permissible.” It is evident thus that during Talmudic times, at any rate, the Isru Hag was not an established custom and that there is no convincing halachic foundation for observing the day after a festival, including Pentecost, as bearing some special significance.
It is noteworthy that neither Alfasi nor Maimonides nor R. Asher make any reference to the Isru Hag. The first of the codifiers to refer to it is the Tur. His reference to Isru Hag is however restricted to the day that follows the festival of Passover and has obviously its source in the Roke’ah.

J. Newman, “Isru Hag” in Essays Presented to Chief Rabbi Israel Brodie on the occasion of his seventieth birthday, eds. H.J. Zimmels, J. Rabbinowitz, and I. Finestein (London: Jews’ College, 1967), 304-305.