harvest, year, age
亗 centers on the cycle of time marked by harvests: a year, age, or the gathering of crops. The meanings are connected through the idea of seasonal completion.
亗 is an ancient variant form of 歳 (year, age). It combines 山 (mountain) and 二 (two), but the exact historical development is uncertain. The modern form likely arose as a simplified or calligraphic shorthand.
Picture a mountain 山 with two 二 harvest seasons stacked below it, marking the passing years and the aging of the land.
For サイ, imagine a sigh of relief at the end of a long harvest year: sigh -> サイ, as the mountain watches over the completed cycle.