Deuteronomy 16:14 "And you shall rejoice on your festival"

The Torah teaches us "Serve HaShem with joy."

This applies to all Mitzvot.

However, during Pesach, Shavuot and Sukkot, happiness itself is a Mitzvah.

Festive meals, fancy clothing, song and dance, are all part of this rejoicing.

This celebration is to be highlighted with the presentation of a sacrifice - Shalmei Simchah - the peace offering of joy.