サイ, まつ.る, まつ.り, まつり
ritual, offer prayers, celebrate, deify, enshrine, worship
JLPT2 G3 S11 F1124
The top element was originally an abbreviated form# of 刷 scrape away impurities. 祭 adds 示 altar/the supernatural → purify unclean meat by scraping away impurities and making an offering of it → *offer prayers*; *worship*; *festival* → *event*.
[Full Kanji Etymology List]