They are playing mind games with you. The time over the weekend will allow you to think & they are hoping you feel a sense of guilt so you come in on Monday & retract the resignation. Type up a new letter of resignation if you like but date it today & deliver it on Monday.
I don't know what your job is size of organisation etc but a good reference is essential. Is there another manager who works there that will give you a good reference?
Just things to consider of course.