Psalm 100 is traditionally omitted, as mentioned by Rashi's student Simcha ben Samuel and discussed in detail by 14th century writer David ben Joseph ben David Abduraham, on Shabbat and festivals because the Thanksgiving offering was not offered on these days in the Temple. Only communal offerings were brought on these days. It is also omitted on the day before Pesach and during Chol HaMoed Pesach because the Thanksgiving offering is composed of a loaf of bread, which is chametz that may not be consumed during Pesach. It also is omitted the day before Yom Kippur because no food is consumed at all on Yom Kippur.