Unless you explicitly specify to the IRA custodian that a contribution made between January 1 and April 15 is a contribution for the prior year, by default the custodian must treat the contribution as a contribution for the year in which the contribution is made. In the case of a contribution made between January 1, 2021 and April 15, 2021 (perhaps now May 17, 2021 due to the extension of the filing deadline for 2020 tax returns), if you did not specify to the IRA custodian that it was to be a contribution for 2020, the custodian will treat it as a contribution for 2021. Regular monthly contributions would probably be contributions for the year in which the contributions are made, but check with the IRA custodian to be sure.
