According to the Walt Disney World Park Calendar Mickey’s Once Upon A Christmastime Parade is going to begin daytime performances from December 22 through December 31, 2021. Mickey’s Once Upon a Christmastime Parade will be performed at 12pm and 3.45pm on December 22 – December 29, 2021. Then on December 30 & 31 it will be performed at 12pm and 3.30pm.
Note that Disney may change showtimes.