One morning my ceiling had a big bubble in it. The landlord found that the sink pipe in the apartment above me had rotted. They replaced it and that seemded to be the end of the problem. But three weeks later it started leaking again. They tried to fix it again. But ever since then it has been leaking, but only about one night per week. What would cause a ceiling to leek only at night, and only one night per week.

What is above you? Sink, shower/tub as in bathroom, or kitchen?

What would cause a ceiling to leek only at night, and only one night per week
If someone above was taking a weekly shower that would explain a lot. The first place I would check is the shower in the upper apartment.
Good luck, Tom