wwosen -
Normally, USCIS tries to have you coming in to Application Support Center the following day.
When I applied for my 2nd EAD online, I filled out the application on Friday and got my appointment at ASC the very next day around 2pm.
as pavan kumar mentioned, don't forget to print out the confirmation notice and take it to ASC. This is very, very important for this is how the people at ASC check for your appointment.
I will make two copies of it, since the person at the ASC took my copy (good thing, I left another copy at home).
Japanese