Short answer:
Very important saints days are called Solemnities and Feasts. Out of the Solemnities, a few are deemed so important that the Church attaches to them the precept of attending Mass and abstaining from unnecessary work. These are the Holy Days of Obligation - you are obliged to attend Mass.
All other saints feastdays are called memorials. However, in order to prevent the calendar from becoming too crowded, some saints have the rank of an optional memorial – you can choose whether you would like to celebrate the saint or not. The rest of the memorials are obligatory memorials. It is obligatory for a priest who celebrates Mass, or someone who recites the Liturgy of the Hours on that day to celebrate that saint, instead of the weekday.
Notice the difference - in one, the obligation is to attend Mass. You have to attend Mass on that day, and most priest have to celebrate Mass on that day. In the other, IF a Mass is celebrated, then the saint must be celebrated. You aren’t obliged to go to Mass on that day. A priest is not obliged to celebrate Mass on that day.
The first case refers to attendance, the second case to the choice of texts.
Longer and pendantic answer:
What does it mean to “crowd” the calendar with feasts? Aren’t saints important? Why should we observe a simple weekday to a saint? Why make some saints optional?
The saints are important – very important. However, the liturgical year takes us through the life of Christ and the different aspects of Christian living. For example, in the season of Advent, we are preparing for Christmas. The Church shows this in many ways – purple vestments are worn and special festive hymns (like the Glory to God in the highest at Mass) are not used. The saints’ feastdays, on the other hand, are joyful occasions. The priest wears white or red vestments, and can use special festive texts. If there are too many saints in Advent, for example, then the idea of rejoicing can suppress the idea of preparation. That loses something of the point of Advent.
There are many other factors that go into making a memorial an optional memorial. Sometimes a saint may not have a very wide devotion, but the Church would like to highlight the contribution of his or her country to the rest of the Church. Thus we have the option of celebrating the saint on a particular day. Sometimes, a certain country or diocese or religious congregation may have a lot of its saints on the calendar. In this case, some of them may be made optional.
