Wrong Day

If the date is correct, but the answering machine has the wrong day of the week, it is because the year is not set properly on the base. 
When a call comes in, assuming you have caller id, the phone receives the month and day from the phone company. But in order to figure out if its Tuesday, Wednesday, etc, the phone needs to know the year.
The phone was shipped with the year set to whatever year the phone was manufactured in - perhaps 2006. Once it is the next year, the user must go into the menu and manually change the year to the correct year. 
On the 7561 I believe you would use a handset, select Menu and go to Setup Devices and select Base and go from there.

Unplug the phone until it gets to that day?