Albert Barnes Commentary Numbers 10:10

"Also in the day of your gladness, and in your set feasts, and in the beginnings of your months, ye shall blow the trumpets over your burnt-offerings, and over the sacrifices of your peace-offerings; and they shall be to you for a memorial before your God: I am Jehovah your God." — Numbers 10:10 (ASV)

In the day of your gladness - Compare Numbers 29:1; Leviticus 23:24; 2 Chronicles 29:27; Ezra 3:10; Nehemiah 12:35, Nehemiah 12:41; Psalms 81:3.