"Rend your heart and not your garments. Return to the Lord your God, for he is gracious and compassionate, slow to anger and abounding in love, and he relents from sending calamity." NIV translation
Even during this great time of trouble, God is still offering His mercy and His grace. The call, here, is for a change of heart to replace the religious rituals. That same call goes out even today as many Christians are going through the motions of religious rituals instead of having a turning of the heart to God.
"Who knows? He may turn and have pity and leave behind a blessing - grain offerings and drink offerings for the Lord your God." NIV translation
This turning of the heart to God will lead to peace with Him. There may also be a physical blessing as mentioned here with the crops producing grain and fruit. A key point is that the prophet says that God MAY "leave behind a blessing". There are those today who have come to faith in a god of their own creation where they can just demand that he give them whatever they want.
Our reward of salvation through Jesus Christ should be enough to satisfy us and anything else that the Lord provides is like a bonus while we are here on the earth. As we see, with this example, even the blessing of the grain and the fruit was to be used to worship Him. Those that are peddling salvation as a means to present God with a shopping list of things that they want are doing so for their own purposes and not for the glory of God. Those that are peddling the notion of a "seed offering" are, in effect, saying that God can be corrupted by a bribe. In both cases, their hearts have not been changed and one must wonder if they have truly repented of their ways.
"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.