irs reg 1.125-4(c)(2)(iii) provides that if employment termination allows you to contribute to HSA provide FSA coverage is terminated.
Under the last-month rule, you are considered to
be an eligible individual for the entire year if you
are an eligible individual on the first day of the last
month of your tax year (December 1 for most taxpayers).
If you meet these requirements, you are an eligible individual
even if your spouse has non-HDHP family coverage,
provided your spouse’s coverage doesn’t cover you.
Also, you may be an eligible individual even if you receive
hospital care or medical services under any law administered
by the Secretary of Veterans Affairs for a service-
connected disability.
You are treated as having the same HDHP coverage for the entire
year as you had on the first day of the last month if you
didn’t otherwise have coverage.
Testing period. If contributions were made to your
HSA based on you being an eligible individual for the entire
year under the last-month rule, you must remain an eligible
individual during the testing period. For the
last-month rule, the testing period begins with the last
month of your tax year and ends on the last day of the
12th month following that month (for example, December
1, 2019, through December 31, 2020).
If you fail to remain an eligible individual during the testing
period, for reasons other than death or becoming disabled,
you will have to include in income the total contributions
made to your HSA that wouldn’t have been made
except for the last-month rule. You include this amount in
your income in the year in which you fail to be an eligible
individual. This amount is also subject to a 10% additional
tax. The income and additional tax are calculated on Form
8889, Part III.
in short if you use the last month rule for 2020 you must stay an eligible individual for all of 2021.
this assumes you are not married
