Okay, understood.
You will need to use a Custom Form for this, so that you can detect that a weekend day has been selected and apply the offer before the form is submitted. Have a look at the thread that I linked to, and you can download a template that demonstrates how to create a custom form.
When a date is selected, to detect whether or not it falls on a weekend, you simply need to pass the date to a Math column that uses the following formula:
Weekday(Date)
The above will return a number between 1 and 7, where 1 is a Sunday and 7 is a Saturday. So assuming that you define a weekend as Saturday/Sunday, then if the math column returns 1 or 7 then you know the selected date falls on a weekend.