You shall observe Lord Caitanya's birth day as follows:
1. The picture of Lord Caitanya with His party should be nicely decorated with flowers and garlands and Sankirtana should be performed regularly from Morning to evening. Just after seeing the Full Moon on the sky the days fast should be broken. I mean the devotees should observe fasting the whole day. In the evening the devotees should take food as in the Ekadasi days.
2. The next day you can celebrate feasting on account of Lord Caitanya's appearance and read about His life as given shortly in my Srimad-Bhagavatam and you can read also from the teachings of Lord Caitanya part of which is going to be published in the present issue of Back to Godhead.