下旬
げじゅん · gejun · JLPT N2 · noun
calendar month
下 (under/below) + 旬 (ten-day period) = the last ten-day span of the month
旬 means a ten-day period, and 下 means lower or later. Together, 下旬 marks the final third of a month, roughly days 21 to the end.
下 — horizontal line 一 + mark below
下 is an indicative character: a long horizontal line represents a reference level, and a shorter stroke placed beneath it points to the area below. The original meaning 'below, down' comes directly from this spatial diagram, and it later extended to verbs like 'descend' and 'lower'.