I think missed a couple of days, so if I'm correct I should have to do days 19, 20, and 21.

Day 19: Favorite Cover Song by Afterglow

ROKI ROKI NO ROCKIN' ROCKIN' ROLL!!

Day 20: Favorite Cover Song by HHW

SHANGHAI HONEY TO HAMABE SHAKOU DANCE MITSUMEPPA NASHI TAMANNEE MEGAMI!! SHANGHAI HONEY TO ASIAN RANDEBUU YOSETE WA KAESU SHITAGOKORO TO MORARU!!

Day 21: Favorite Cover Song by Pastel✽Palettes

I'll have to go with Nostalgic Rainfall. It just sounds so good compared to a lot of other PasuPare covers imo.