1The first third of a month, roughly the 1st through 10th, used to indicate an approximate date range.
来月の上旬に引っ越しをする予定だ。
I'm planning to move at the beginning of next month.
桜は例年、4月上旬に満開になる。
Cherry blossoms usually reach full bloom in early April.
今月の上旬はずっと雨が続いていた。
It kept raining throughout the beginning of this month.