Usually, the IRS will withdraw the amount on the day you selected. If that date happens to fall on a weekend or federal holiday, the funds are typically withdrawn the next business day. But there seems to be some delay with the IRS withdrawing the funds. Please be aware that this year you have until May 17th to pay your balance due. Please see When will my tax payment go through? for additional information.
Also, you can view your balance and check your payment activity at IRS Paying Your Taxes.