Both "for" and "on" can be used. I would usually use "on" but would be more likely to use "for" if the time slot filled the whole day or most of the day. It was quiet out in the suburbs. ("out" suggests a comparison with the region that is "in". In this case, that would be in the city.)

There is a difference in meaning. Example: On the 18th, we called and scheduled a time slot for the 20th. That is, we called and scheduled on the 18th, but the meeting will take place on the 20th. “On” refers to the day an action occurs, but “for” refers to the day an action is going to occur based on a schedule. We scheduled on the 18th for the 20th. Let me know if you've understood my explanation.

"for" is used often for scheduling and planning, as it suits the sense of destination, be it a place or time. Just like a train might be bound for a certain destination, an event can be scheduled/set for a certain time. "I scheduled a time slot ON the 20th" is also okay, but note that it is possible (albeit not likely) to interpret this sentence as "I did the act of scheduling on the 20th" and not as a statement about the scheduled date. "for" has no such room for ambiguity, so it might be a better choice.

Phi is absolutely right.

[edited - I agree with Phil.] The general answer is "at" a time, "on" a day/date, and "in" a period of time such as a month, season, or year. "To schedule for" a time, a day/date, a period of time is correct. I use a mix of "schedule for" and "schedule at/on/in." The Collins dictionary is a good resource. https://www.collinsdictionary.com/dictionary/english/schedule verb [usually passive] If something is scheduled to happen at a particular time, arrangements are made for it to happen at that time. A presidential election was scheduled for last December. [be VERB-ed + for]