Your 2019 federal tax due must be paid by July 15 2020.
If you have federal tax due you can pay by mailing your payment with the 1040V voucher, having the payment taken out of a designated bank account, or you can pay directly on the IRS website.
If you are mailing your check along with your tax return, you don't need a voucher. Put the check in with the return and mail them together.
There is no way to know when the IRS will actually cash the check since they are way behind processing mailed tax returns. If you prefer to know when the funds will be taken from your bank account, you would be better off using IRS direct pay to schedule the debit from your bank account instead of mailing a check with your return: https://www.irs.gov/payments/direct-pay