en_tn/1ki/12/33.md

went up to the altar

"Went up" here is an idiom for going to a sacred place to worship since these altars were located on high places. Alternate translation: "offered sacrifices on the altar" (See: rc://en/ta/man/jit/figs-idiom)

in the month he had planned in his own mind

"in the month that he had determined"