Her due date was on Sept 18th? Did you witness the breeding and have calculated correctly? Is she acting like she may be in beginning stages now? On Monday, if she has not started labor, contact your vet and fill him in on the situation. He may want to see her.
In general they can be expected to go 5 days before or 5 days after their due date and anywhere in between, lol. I have had them go as long as 6 days late, but that is not ideal. Your vet will likely want to induce or c-section her if he has not done anything by Monday. The problem with the toy breeds going so late is the growth rate of the pups is very high at the end of the pregnancy, and can easily get too big for her to pass if let to go too late.
I am generally on once through the day for a short time, and not back until later in the night, I have a zoo right now that keeps me busy, lol.
There are some other here too that are experienced in whelping, not all in Min Pins, but they have excellent advice as well.
Hope it all goes well, I will look for you to be on to see if you have any questions and can give you my e-mail if need be :)