Sometimes called "Dharma Day", Asalha Puja commemorates the first sermon of the Buddha. This is the Dhammacakkappavattana Sutta, meaning the sutra which set the wheel of dharma in motion. In this teaching, the Buddha explained his doctrine of the Four Noble Truths.
Vassa, the Rains Retreat, begins the next day. During Vassa monks and nuns remain in monasteries/temples/hermitages and intensify their meditation practice. We can leave for important duties. Therefore, I will be mostly unavailable during this time.
Laypeople can visit and participate by taking food, candles and other necessities to monks/nuns. They also sometimes give up eating meat, smoking, or luxuries during Vassa, and take extra effort to practice the Five Precepts.