On the morning after the disastrous dinner with the Haydens, Lorelai suddenly remembers that she never showed up to help Luke paint the diner on Friday night. She runs over while still in her pyjamas to apologise and explain, unnecessarily increasing Luke’s pain by letting him know that she was with Christopher.
In fact their “date” for painting the diner was so vaguely worded that I couldn’t tell when exactly they were planning to do it, and Lorelai had triple-booked herself, as she had Friday Night Dinner with her parents, and also business class, which she never seems to attend. Even if Emily let Lorelai leave early, when were she and Luke planning on doing the painting – in the middle of the night?
As their poorly-planned painting date didn’t happen, Lorelai somehow manages to sneak into the diner while Luke is asleep – the bread guy let her in, suggesting she started at dawn – and paints the entire thing in an hour or two. This would take at least two days (you can’t just start painting, walls have to be prepped), and she was doing it all alone.
Furthermore, later episodes confirm that Luke slept over the diner and would have almost certainly heard her, and that he got up extremely early so the diner could open at 6 am, meaning that there was no time for Lorelai to paint the diner before he got up (and also that he couldn’t have possibly painted the diner at night as they originally planned; he wouldn’t have got any sleep).