They go to mass each week and receive Holy Communion.
It was the last time that Jesus spoke with his disciples before the Crucifixion, and he showed them how to consecrate the Eucarist.
The day before Good Friday, also known as Maundy Thursday. I don't know about all Christians but many replicate the communion that they had at the last supper and some people have a meal before doing...
Of course, Catholics celebrate Christmas. It is one of the three greatest feasts of the Liturgical Year.
The Eucharist
This depends on what religion you are. I am a Roman Catholic, therefore I celebrate it on the Thursday before Easter.