All events trigger at random. The marriage event will not be able to be triggered until the player is currently dating a candidate, have gone on a minimum of three dates, have seen the sub-event, and have their LP at 10 or more. LP will increase much slower from 7 points up if you are not dating a person. When you have multiple boyfriends / girlfriends, it's best to try to confess at 10 LP.
You can give 1 neutral, 1 liked, and 1 loved gift to each person everyday.