> I recently set up a sale for November and set it to 1/11/2021 to 30/11/2021 the sale ended a day early on the 29th of November.

An end date of 30/11/2021 (or 11/30/2021 for Americans) means the sale or special ends on that date at time 00:00:00. So one second after midnight on 11/29, the sale ends if the end date is 11/30.