These may vary. The boy generally wears maybe a suit, or just slacks and a shirt. When a child is seven or eight, they can make their Communion. In school the class would prepare for this in the religious education lessons. The ceremony consists of a normal mass and then each child having their first taste of communion.
Communion is like a small round bit of wafer-like bread.
It is sometimes called the host. Catholics believe it became the risen Jesus. Here is the Roman Catholic Calendar. It notes most of the very important Roman Catholic feasts and festivals. The dates of lent and Easter change each year.