上旬, 中旬, and 下旬: three parts of a calendar month
Read days 1–10, 11–20, and 21 through the month’s end, allowing the final part to vary with the calendar month.
The first part of a month
上旬, read じょうじゅん, covers the first through the tenth. 四月上旬に届く予定です。 says that arrival is planned for the first ten days of April.
上旬 does not by itself specify April 1. An exact date requires more specific wording, such as 四月三日.
The middle ten-day part
中旬, read ちゅうじゅん, covers the eleventh through the twentieth. 六月中旬に集まります。 says that the gathering takes place in the middle part of June.
六月十五日 belongs to 中旬. 六月中旬, however, does not by itself fix the date as the fifteenth.
From the twenty-first to the month’s end
下旬, read げじゅん, covers the twenty-first through the last day of the month. 四月下旬 therefore covers April 21 through April 30.
二月下旬 starts on February 21 and ends on that year’s last day of February. Its length depends on whether the month ends on the twenty-eighth or twenty-ninth; it is not always ten days.
Does 二月下旬 end on February 30 in every year?
Show the answer and why
No. It ends on the actual last day of February that year, either the twenty-eighth or the twenty-ninth.