Translation:Pentecost is fifty days after Easter.
It's a Christian holiday, marking fifty days after Easter, when Christians believe God sent the Holy Spirit down on the disciples. It's often considered the birthday of the Church.
I wrote Whitsun too and it was not accepted. Although I knew the term Pentecost I would go so far as to say that 80% of UK English speakers would never use this term.
Pentecost is not a noun that uses 'the' in English. It would be like saying 'The Christmas', 'The Easter' or 'The Halloween' (all of which don't take 'the' either).
Hvorfor skriver I så ikke : Pinse er halvtreds dage efter påske? Hvorfor pinsen?
I was actually kind of hoping that Pentecost would have a name related to the unique Danish numbering system... would've made it easier for me to remember the numbers!