The expression “Yom Tov” literally means “a good day.” Nowadays it can be used as its literal expression suggests, such as in the blessing, “Have a good day.”
Biblically, “Yom Tov,” Good Day, is a name for a fixed date in the calendar, which the Torah has set as a holiday.
The Torah has set six Good Days:
The First of Tishrei: It is Rosh Hashanah (the New Year), which is called in the Torah, “Yom Te-ruah (a day of blowing the horn). It is also called “Zich-ron Te-ruah” (a memorial of blowing of horns).
The Fifteenth of Tishrei: the first day of the Feast of Sukkot (the Feast of Tabernacles).
Twenty-second of Tishrei: Simchat Torah, which is called in the torah “Atzeret” (rally).
The Fifteenth of Nissan: the first day of the Feast of Passover.
The Twenty-first of Nissan: the seventh day of Passover.
The Sixth of Sivan: On the fiftieth day of the counting of the Omer – Shavuot, Feast of Weeks, or Pentecost. This Feast is called in the Bible “Bi-ku-rim,” “the day of first fruits.”
Due to the method of setting the Hebrew months in the past, there was no certainty for Jews who lived abroad of exactly when to celebrate the Good Days (they did not know whether the court intercalated the month or not.) Therefore, the sages determined that every Jew living abroad should celebrate two Good Days on every Good Day holiday (in case you ever wondered why Jews outside Israel celebrate two “Seders” on Passover).