The Last supper occured during the Passover Meal, so in the fact that it was a Passover Meal, it was something that the Jews did once a year. However, the evangelists are clear that this Passover Meal was different than the others. To gain some understanding, do some research of the Passover Meal and how it is conducted.
The term “the breaking of the bread” was a phrase that meant the Eucharistic Sacrifice and Meal. Whoever the two disciples were, they must have either been present at the Passover Meal, or it was something they recognized from sharing with the Apostles.
It should also be noted that while the Evangelists place the first Eucharist at the Passover Meal before Christ’s death, He had talked about it and alluded to it for some time before (e.g. see, John 6). It wasn’t as if this simply came out of nowhere. He gave plenty of lead before the Meal.