上旬
じょうじゅん · joujun · JLPT N2 · noun
month
📅 上 (first, upper part) + 旬 (ten-day span) = the first ten days of a month
A month splits into three ten-day spans: 上旬 (upper, first), 中旬 (middle), and 下旬 (lower, last). 旬 always means a ten-day period, so 上 marks the earliest one.
上 — horizontal line above a baseline
上 is an indicative character (指事): a short stroke placed above a longer reference line shows the concept of 'above' or 'up'. The earliest oracle-bone forms used two horizontal lines for this; the modern shape added a vertical stroke for clarity. The core meaning of 'upper, above, to ascend' has stayed stable for millennia.