The extended light won't bother it for one day. I took care of a friend's seedlings last year when she was away for four days. They were under lights without timers. She wanted the lights left on the whole time. The plants seemed fine and some were tomato plants which she gave me later in the season. They were nice, healthy plants.
I would only give it a short dark period, then go back to your usual schedule and don't worry, it will be fine. I hope your appointment went ok