If its scheduled for April 15th, thats when it will be debited.  You can call in and cancel the payment and use a 1040V to pay in later, or the client can use IRS Direct Pay and can schedule a payment up to 30 days prior to the date they'd like to have it withdrawn.

Call IRS e-file Payment Services 24/7 at 1-888-353-4537 to inquire about or cancel your payment