Matsya means fish, just like he took his other incarnations for the welfare of the creation, in the same way this form of God was also for the protection of the world.
The third day of the Shukla Paksha of Chaitra month is celebrated as Matsya Jayanti.
This day is dedicated to the Matsya incarnation of Shri Hari Vishnu. Special worship of Vishnu ji is done on the day of Matsya Jayanti.