A Chag (חַג), Strong's Number H2282,is the Hebrew noun meaning “feast” or “festival”. It's used to describe feasts such as the Holy Days of the Bible, or a sacrifice offered at that time.
Chag is most often used to refer to one of the Biblical Holy Days such as the “Feast of Unleavened Bread” or the “Feast of Tabernacles”. Below are a few examples, where we have substituted the Hebrew word chag in place of the English word that it was translated from:
The chag of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, in the time of the month Abib: for in the month Abib thou camest out from Egypt. (Exo 34:18)
Speak unto the children of Israel, saying, The fifteenth day of this seventh month shall be the chag of tabernacles for seven days unto YHVH. (Lev 23:34)
Chag can also refer to a “feast” that's not specifically one of the days listed in Exodus 23. For example:
And Jeroboam ordained a chag in the eighth month, on the fifteenth day of the month, like unto the chag that is in Judah, and he offered upon the altar. So did he in Bethel, sacrificing unto the calves that he had made: and he placed in Bethel the priests of the high places which he had made. (1Ki 12:32)
A chag is also a sacrifice offered during the time of a feast. Here are a few examples:
Thou shalt not offer the blood of my sacrifice with leavened bread; neither shall the fat of my chag remain until the morning. (Exo 23:18)
God is YHVH, which hath shewed us light: bind the chag with cords, even unto the horns of the altar. (Psa 118:27)
It is sometimes argued by Lunar Sabbatarians that the chag described in Exodus 13:6 cannot be the seventh day of Unleavened Bread, in part because the word chag is only used to refer to the pilgrimage feast days listed in Exodus 23:14-17. It's also often argued that the Fourth Month Feast led by Aaron at Mt. Sinai had to be the feast of Pentecost because Aaron declared it to be a "chag to YHVH" (Ex 32:5).
Unfortunately neither of these assertions are true, as demonstrated by the scriptures listed above. A chag can refer to many other days besides the three pilgrimage festivals.