A pun is a humorous use of a word that has two meanings, or of words with the same sound but different meanings. For example, the joke 'When is a door not a door?' 'When it's ajar' depends on the fact that 'ajar' sounds like 'a jar'.
In the early Roman calendar, April was the second month but became fourth when they started to use January as the first month. The origin of it was Aprilis, meaning 'to open'. In April, small animals that were hibernating usually come out and the birds fly back northward to settle, bees and butterflies begin to gather nectar. In some parts of the world, April is planting time but in other parts, it's the harvest season.