Ok. Waouh I don't know anything about that ! I have so much to learn !
Apparently, Saturn was opposite my Sun (1,1Ā°), square my Natal Jupiter (0,15Ā°). I'm Sagittarius rising so Jupiter is my chart ruler. Does this means this transit was important ?
Indeed it does, and not just because Jupiter is your chart ruler, although that's certainly a consideration. Long term relationships usually start when there's both a significant Saturn and a significant Jupiter transit, and/or transits to those planets in the natal, going on. It takes Jupiter to get you interested and Saturn to get you committed.
Saturn was also making trine to certain planets, does it count ?
I would consider those, although it's the hard angles that tend to drive change. Starting a new relationship is a change. Trines are some smooth flowing energy--problems that have already been solved, changes that have already been made. For example, suppose you have transiting Saturn trine your Moon, and the last hard transit Saturn made to your Moon was a square. At that square, something would've come up for you that required you to make a decision and/or a commitment (Saturn is all about commitment--one reason it's going to be up when a serious, lasting relationship starts). At the trine, you're coasting on the momentum of that decision or commitment.
My boyfriend had Saturn square natal Midheaven (1,19Ā°) and square Mercury but with an orb of 5Ā°
The square to the midheaven was definitely in play. With Mercury, that's a wide orb for a transit to be felt. It wasn't necessarily insignificant for him at that time, but probably less significant than when the square was within 2 to 3 degrees of exact.
However, if he has a natal aspect involving his Mercury, and Saturn was in closer orb to an aspect with one of the other planets involved, that could've brought Mercury in through that aspect.
