When and how is Mass celebrated at a side altar? Was it just used for Low Mass in the past, then High Mass or Sunday Mass was celebrated on the main altar?
For side altars that are on the side walls, isn’t it rather awkward to have Mass there since the seats don’t face in that direction? Also, during the pre-Vatican II days didn’t it conflict with the rule about celebrating Mass facing east? (Presumably the High Altar would face east; thus altars along the side walls would face north or south – or was that even an actual rule or just the recommended norm?)
There was no rule requiring one to face the actual East.
The side altars could be used for Low Masses; however usually there could be only one sung Mass at a time in a church. In addition, there were certain rules affecting certain kinds of Masses and times during which it was forbidden to celebrate Mass at a side altar (e.g. certain Pontifical Masses, the conventual Mass [for certain clerics], the Triduum, and so forth)
Today side altars may be used in conformity with the GIRM:
An individual priest is, however, permitted to celebrate the Eucharist individually, though not at the same time as a concelebration is taking place in the same church or oratory. On Holy Thursday, however, and for Mass of the Easter Vigil, it is not permitted to celebrate individually.