It's an order!
"Christ, our Passover lamb, has been sacrificed. Therefore let us keep the Festival..." 1 Corinthians 5. 7-8
Maundy Thursday derives its name from the Latin "mandatum" meaning "order/ instruction". At the Last Supper Jesus instructed his disciples to remember what he was about to do - sacrifice his perfect life on the cross - through bread and wine. Remembering Jesus in breaking bread and sharing wine has, of course, become the heart of our Communion celebrations, whether they are occasional and informal or elaborate and spread over a weekend.
Holy Communion takes its place alongside Baptism as one of two "sacraments" acknowledged and celebrated by all Christians of whatever denomination. The 4th century African theologian, Augustine, regarded occasions such as Easter, Ascension and Pentecost as sacramental also because of their significance for us, personally. In so doing he distinguished these "sacramental" events from the likes of Christmas because, while important, such celebrations do not impact directly and personally on our lives in the same way.
That is not to denigrate some festivals as "lesser" but to emphasise the personal significance of those pivotal celebrations which the Lord commands us to enter into, in order to mature and flourish in our faith and in our discipleship. So, let us keep the Festival and enjoy the benefits of this Maundy Thursday and give to God all the glory...