The answer depends on what type of transaction is used to move the funds from the traditional IRA to the Roth IRA. If you were eligible to have made the contribution originally as a contribution to a Roth IRA for 2020 and simply want your traditional IRA contribution to have been a Roth IRA contribution instead, you would do a recharacterization. The deadline for doing a recharacterization is the due date of your tax return.
If you instead want to move some other funds to the Roth IRA or are not eligible to make a Roth IRA contribution for 2020, you would move the funds by doing a Roth conversion. A Roth conversion is reportable on the tax return for the in year in which the distribution from the traditional IRA occurs. If you do a Roth conversion now, you would report it on your 2021 tax return; it would have no effect on your 2020 tax return.
I suspect that it's a Roth conversion that you are asking about, not a recharacterization.
