The seasonal significance of the winter solstice is in the reversal of the gradually lengthening nights and shortening days. How cultures interpret this is varied, since it is sometimes said to astronomically mark either the beginning or middle of a hemisphere's winter. Though the winter solstice lasts an instant, the term is also colloquially used to refer to the full 24-hour period of the day on which it occurs.
The word solstice derives from Latin sol, meaning 'sun', and sistere, 'to stand still'.
Saint Thomas' Day is also celebrated on 21 December. Saint Thomas is commemorated on this day because he was the last one of the apostles to become convinced of Jesus' resurrection - in other words, he was the one who for the longest time remained in the 'night of unbelief and doubt.' He is also supposedly to have died on this day c. AD72, near Chennai in India.