Background of Pesach Sheiyni:
In the times of the Temple, a person who did not bring the Pesach offering in the 14th of Nissan, either due to being impure or due to being a distance from Jerusalem, was required to bring the offering a month later, on the 14th of Iyar.
Increasing in Simcha:
One is to increase in joy out of the holiness of the day.
Learning the laws of Pesach Sheiyni:
It is proper to learn the laws of Pesach Sheiyni on the 14th of Iyar.