下旬【げじゅん】
last third of a month, 21st to the last day of a month
中旬【ちゅうじゅん】
middle of a month, second third of a month, 11th to 20th day of a month
上旬【じょうじゅん】
first 10 days of month
初旬【しょじゅん】
first 10 days of the month
旬【じゅん】
ten-day period (in a month), ten-year period (in one's age), decade