Sometimes called “Dharma Day,” Asalha Puja commemorates the first sermon of the Buddha. This is the Dhammacakkappavattana Sutta, meaning the sutra (sermon of the Buddha) “setting the wheel of dhamma [dharma] in motion.” In this sermon, the Buddha explained his doctrine of the Four Noble Truths.
Vassa, the Rains Retreat, begins the day after Asalha Puja. During Vassa, monks remain in monasteries and intensify their meditation practice. Laypeople participate by bringing food, candles and other necessities to monks. They also sometimes give up eating meat, smoking, or luxuries during Vassa, which is why Vassa is sometimes called the “Buddhist Lent.” (http://buddhism.about.com/od/buddhistholidays/tp/buddhistholidays.01.htm)