Assuming that you have qualifying HDHP insurance on the first day of December, 2020, and assuming that your Medicare effective date is in 2021, then you and your spouse are entitled to make full year HSA contributions for 2020. If you are not covered by a qualifying HDHP plan on December 1, then you only have 11/12ths eligibility for 2020.
I am not an expert on Medicare effective dates, but if the effective date is retroactive to 2020, you will have to remove the excess contributions from your HSA before April 15, or pay a significant penalty.
