Oh, no, no, no. We don't "always" do much of anything when it comes to prepositions. It depends on the surrounding words.
In the situation of primary scheduling, you need to know your available resources. (otherwise, your schedule may not be accurate)
At the time of primary scheduling, we had more resources available. (now, some people have been laid off, and we need to develop a new schedule)
Both of those are fine, although I'd probably reword the first:
During primary scheduling, you need to know your available resources.
I recommend looking up those pesky little English words we call prepositions – like of, in, to, for and at – in a good dictionary, such as Collins or Macmillan. Study the dozen or so definitions you'll find for each one, and notice how vague some of those definitions are (like, "concerning or showing someone or something"). You should come to two conclusions:
- No one is going to be able to briefly and concisely explain when to use in and when to use at.
- There may be some instances where either one could be used, because the possible meanings of the two words overlap.