>Do you know how the day system used to work when there were only 20 mysteries?

Yes, I do.
On Mondays and Thursdays, pray the Joyful Mysteries.
On Tuesdays and Fridays, pray the Sorrowful Mysteries.
On Wednesdays and Saturdays, pray the Glorious Mysteries.

As for Sundays - during Advent and Christmas time, pray the Joyful Mysteries.
During Lent, pray the Sorrowful Mysteries.
All other Sundays, pray the Glorious Mysteries.

The Rosary in the form that our Lady gave it to St. Dominic consists of 150 Hail Marys and 15 Our Fathers. When John Paul II added the Luminous Mysteries, he changed the chaplet into something other than the true Holy Rosary as the Mother of God herself defined it.

N.B., I am a Traditional Catholic, I do not recognize the innovations of Vatican II, rather I stand with Pope St. Pius X against modernism. I invite all Catholics reading this to explore Catholic tradition and reclaim their spiritual heritage.