749. Honoring Yom Tov

102:7 If yom tov falls on Thursday and Friday and a person realizes on Thursday that he did not make an eiruv tavshilin before yom tov began, he can make one that day. He says the bracha (according to some opinions – Mishnah Brurah 527:74), followed by, "If today is holy, then I don't need to make an eiruv and if today is a weekday, then let this eiruv enable me..." One may not, however, do this on Rosh Hashana, which is legally considered one long day.

103:1 Just as it is a mitzvah to honor and to enjoy Shabbos, so is it a mitzvah to honor and to enjoy yom tov. This is evident from Isaiah 58:13, "(call) the holy day of Hashem honored." The Torah refers to each of the Festivals as "a holy gathering" (see Exodus 12, Leviticus 23, Numbers 28).