For Christians, this festival (one week after Pentecost or Whitsun) honours the three-fold nature of God.
Outline script for assembly leader
Pour some water into a tray. Freeze it. It turns to ice. Now in a different form, it is still the same substance as we can see when it melts and turns back to water. Now boil it. It turns to steam. Again it is in a different form but still the same substance - as we can see when the steam hits a cold surface and condenses back to water.
When Christians say God is "three persons in one being", they are stating their belief that God has shown (or manifested) himself in three ways. They believe that God is revealed in the Bible as a spirit, living for ever.
They also believe that he took human form as Jesus Christ to show that he was not just a distant being but a loving father who understands and has experienced what it is to be human.
Then, 40 days after being crucified and returning to life at Easter, they believe Jesus ascended to heaven. Ten days after that (at Pentecost), God showed himself in a third way. As the Holy Spirit, he came to the followers of Jesus to show that God would always be with them "in spirit". Another name for the Holy Spirit is Comforter which originally meant "strengthener".
Trinity Sunday, when Christians honour the threefold nature of God ("the Trinity"), was not celebrated until at least the year 800. It gained special importance in England after the assassination of Thomas ... Becket in 1170 because he had been made Archbishop of Canterbury on or near Trinity Sunday.
When making the sign of the cross on their forehead and chest, Christians traditionally use three fingers and say "In the name of the Father and of the Son and of the Holy Spirit." A more superstitious belief ascribed great powers to the weather on Trinity Sunday. "Trinity rain" was considered as healthy as "Trinity sunshine".
St Patrick allegedly used the three-leafed shamrock or trefoil to teach the doctrine of the Trinity. Search for other three-leafed plants which could be so used.
Research and define words derived from the "tri-" root: tricycle, tripod, etc.
Design a poster or shield explaining the Trinity using symbols such as the shamrock, triangle or kaleidoscope.