Filed for an extension w/ a payment due. It is now the day I selected for the payment to withdraw, and I do not see it out of my checking. When will it occur?

It depends on the IRS, However, you will not owe a penalty since you have submitted your extension request and set up the payment before 04/15/2019. Please view the additional information below related to payment processing.

https://ttlc.intuit.com/replies/3299924