why do we celbrate it?
ב"ה Shalom, On the one hand, Yom tov is like any other commandment in the Torah, tefilin, mezuza, shofa, Shabbat and keeping kosher. . We are commanded in the Torah and therefore in order to fulfill the will of G-d, we do as G-d has commanded. However, Yom tov does not just have obscure reasons as some mitzvot do, but they have much more obvious and clear reasons. They are therefore also called "Edot", which means "testimony" since the celebration of some of the festivals come to commemorate and testify to things of the past. Such as Pesach and sukkot which are festivals which clearly celebrate our redemption from Egypt and G-d protecting us in the desert after the exodus. On Shavuot, we remember the receiving of the Torah and hearing the Ten commandments. Rosh Hashanah is a day of judgement, and Yom Kippur is a day of Atonement. Each one of these holidays, had their special ways of commemoration during the time of the Bet Hamikdash and what remains today are the special prayers, reading of the Torah, the festive meals and the prohibition of doing different actions of "melacha"- of creative work. Then we have the holidays which were instituted by the Rabbis such as Purim and Chanuka, which are not written in the Torah. In order to give you a more comprehensive answer, please be more specific in future questions. All the best