During non-peak times, funds are usually withdrawn on the payment date you specify, assuming your e-filed return has already been accepted (received) by the IRS.
If that date happens to fall on a weekend or federal holiday, the funds are typically withdrawn the next business day.
Due to high e-filing volume it may take a few days for the payment to be debited from your account. In addition, it can take your bank another few days to post the debit to your account.
No worries; as long as you filed your return or extension on or before April 17, 2018 and you selected April 17, 2018 (or earlier) as your payment date, you won't get penalized for late payment.
In the meantime, keep an eye on your bank account – if you still don't see the debit 7-10 days after your return has been accepted, call IRS e-file Payment Services at 1-888-353-4537 for assistance in verifying the scheduled debit date.