Joel 2:15 & 16
"Blow the trumpet in Zion, declare a holy fast, call a sacred assembly. Gather the people, consecrate the assembly; bring together the elders, gather the children, those nursing at the breast. Let the bridegroom leave his room and the bride her chamber." NIV translation
Now, we see the focus of the Day of the Lord which is on the Jewish people. The trumpet was first used to call the Jews to assemble while they were in the wilderness (see Numbers 10). The holy fast (no eating or drinking) was first given to the Jews for the Day of Atonement (see Leviticus 23). The sacred assembly was first given to the Jews as part of the celebration of Passover (Feast of Unleavened Bread, see Exodus 12). As we see in this passage, all of the people were required to gather from the smallest child to even the newly married.