הלכה על ברכות 98:11

A person must mention the holiness of the day on Shabbat and holidays in the third blessing, like we said. And if he did not mention it and he began 'The One who is good and does good,' he goes back to the beginning (Mishneh Torah, Laws of Blessings 2:12). If he did not begin it but he did finish the third blessing, he says this formula on Shabbat: "Blessed are You, Lord, our God, King of the Universe, who has given Shabbat rest to His people, Israel, as a sign and a covenant. Blessed are You, Lord, who sanctifies the Shabbat." And if it is a holiday, we say, "Blessed are You, Lord, our God, King of the Universe, who has given holidays to His people, Israel, for happiness and joy. Blessed are You, Lord, who sanctifies Israel and the times." And so [too,] Rosh Chodesh (the first day of the month), the intermediate festival days, Channukah and Purim have a mention in the Grace after Meals in the third blessing. But if he forgot [in these cases] and concluded the blessing, we do not make him go back, and we do not mention [the holiday] at all. I have received as a tradition from my teachers, may God protect them, that any one who is careful about Grace after Meals will have his food available with dignity all of his days. [These] and the rest of its details are elucidated in Tractate Berakhot (see Tur, Orach Chaim 188).