When is the deadline for contributions to an HSA for any particular year?

Total contributions for the year can be made in one or more payments at any time up to the tax-filing deadline, without extensions. (This is generally April 15 but for the 2019 tax year, due to COVID-19 relief act, the deadline is July 15, 2020.) However, if the individual wishes to have a contribution made between January 1 and April 15 treated as a 2019 contribution (for the preceding taxable year) the individual must take one of these steps to notify UMB, so that UMB can report it correctly to the IRS:

In the absence of one of these three steps, all contributions from January 1 through July 15 will be treated as contributions for the 2019 taxable year.

Contributions are only accepted in the form of cash. Cash Payments can be in the form of a check, online contribution or employer contribution. No other forms of payment (such as stocks or property) are accepted for HSA contributions.