Psalms 81:3-5

He called on them to participate in a festival. The Israelites blew trumpets and offered sacrifices at the beginning of each new month, and each month began with the new moon (Numbers 10:10; Numbers 28:11-15). The Feast of Tabernacles was a joyous occasion that began on the fifteenth day of the seventh month (September-October) when the moon was full (Leviticus 23:34). God required the Israelites to observe these occasions. He began to specify these national festivals when He gave the Israelites instructions concerning the Passover (Exodus 12). Back then, this instruction was completely new to the nation, as though it was a voice they had never heard before.