Amos 4:4

God gives them up to their self-willed idolatry, that they may see how unable their idols are to save them from their coming calamities. So Eze 20:39. Beth-el-- (Amo 3:14). Gilgal-- (Hos 4:15; Hos 9:15; Hos 12:11). sacrifices every morning--as commanded in the law (Num 28:3-4). They imitated the letter, while violating by calf-worship the spirit, of the Jerusalem temple-worship. after three years--every third year; literally, "after three (years of) days" (that is, the fullest complement of days, or a year); "after three full years." Compare Lev 25:20; Jdg 17:10, and "the days" for the years, Joe 1:2. So a month of days is used for a full month, wanting no day to complete it (Gen 29:14, Margin; Num 11:20-21). The Israelites here also kept to the letter of the law in bringing in the tithes of their increase every third year (Deu 14:28; Deu 26:12).

— Jamieson-Fausset-Brown Commentary (public domain)