We use the preposition para (not por) when we want to say "when something takes place/took place/will take place" if we refer to a specific event.
Read and listen to these examples:
In the three examples above, by using preposition para, we are expressing the precise time or on which precise event/day something takes place.
Have a look at the these same sentences, one with para and the other with por and see how the meaning changes even though the direct English translation remains the same:
David vino para la celebración. (David came for the celebration.)
David vino por la celebración. (David came for the celebration.)
The first example with para means that David came on the day of the celebration, while the second one with por means that David came because of the celebration. See Using por (not para) to express the originating cause or reason for more information on this last usage.