There isn't a published deadline for the IRS to process your bank debit - they will debit your bank account according to their internal procedures and your bank will process that debit according to their internal procedures.
All you have to make sure of is that there are sufficient funds in your account until the payment is posted.
The IRS can take as long as it likes to debit your account for taxes due them - they can't debit any earlier than you authorized, but they aren't obligated to do it on the day you select. Also, banks do not post bank credits or debits to your account on weekends or holidays.
