It is narrated that Imam al-Baqer (peace be upon him) said that his father Imam as-Sajjad (peace be upon him) said:
"On the first day of Shawwal, a caller will announce: Oh believers, receive your gifts!"
Then the Imam said: "Oh Jaber, these gifts are not like the ones given by kings, they are gifts of Allah."
A divine reward to the believers for their good deeds during the holy month, and thus it is our duty to thank the Lord for his gifts and accept His reward by completing our worship and reviving this day with thanks and getting closer to God.
Source: Alkafeel.net