It's any day of the week. According to the guidebook it depends on the number of days after the last camper left before a new one appears and you have a 20% chance maximum for a camper.
1 day after a camper has left - 0%
2 days - 0%
3 days - 5%
4 days- 10%
5 days - 15%
6 days - 20%