My husband has a solo 401k. He made a contribution for 2019 in February 2020. Do we enter the contribution on 2019 tax documents or 2020?

[ColeenD3]

It depends on his status.

Employee contributions generally must be made by the end of the calendar year, but you have until the tax-filing deadline to make employer contributions.

[dmertz]

The election by the individual to make the employee contribution must be made in writing by the end of the year, but the actual deposit of that contribution can be made early the following year.  If the contribution was an elective deferral for 2019, it must be included on the 2019 tax return.