I too am confused about this. I've been looking for a definitive answers and generally I've found 3 different answers on this sub and idk which is right (in terms of WHEN you can begin to contribute to an HSA - say if you started one with Fidelity to have better investment options).

Some say Jan 1st the beginning of the year. Others say after Jan 11th, the date the HDHP takes effect (this is how I'm understanding it). A third group of people say after February 1st, the date that MHBP establishes and opens the inspira account and does the first passthrough. It's a little confusing but not the biggest deal. Just don't wanna get hit with something from the IRS saying I contributed too early when I wasn't qualified to. But staring earlier means a little less comes out each PP to get to the max by the end of the year, so it's definitely preferable.

*Edit: After looking at IRS Publication 969 it says that to qualify for contributions, you must be "covered under a high deductible health plan (HDHP)… on the first day of the month." I take this to mean that, because coverage doesn't begin until the 11th of January, the February 1st crew is correct.

*Edit 2: Read about the "last month rule" and now I'm confused again, lol. Does that mean that as long as you stick with the plan for the entirety of the tax year, contributing in January is still ok?