Question
I will be away for three weeks beginning in mid-October. I live near Philadelphia, and the temperatures will be in the 60's during the day and the high 30's at night. I mow roughly an acre with a push mower, and I'm reluctant to call a mowing service to mow for me while I'm away. Would it be all right to leave the lawn uncut for three weeks (it may be closer to four) at this time of the year?

Answer
I do not know.
If the grass grows rapidly while you are away, you will have a problem to get through the area and disposing of the clippings mowing with a walk-behind mower. Any weeds will set seeds in that time and you can set yourself up for a weed problem later.
I firmly believe in mowing at weekly intervals at a constant height, so I would bite the bullet and call a mowing service.