If covered call option get assigned after market close 12/31/2020 expiration date, does this report to 2020 or 2021?

[Wood8]

I have a covered call option that is currently in the money and expiration date is 12/31/2020 (last day of the year). If it will get assigned after market close on 12/31, few queries I have:-

1. when will it get assigned, on 12/31 right after market close or 1/1/2021? 

2. does the gain/loss of such option assignment count to 2020 tax year ? or 2021? 

Your help is greatly appreciated. 

[fanfare]

It takes two business days to settle the transfer of stock sold.

BUT, look at your consolidated 1099-B in February and just go with what the broker reports.