adverb, noun
first ten days of a month; early part of a month
Refers to roughly days 1 through 10 of a month. Used as a noun and adverbially in expressions such as 4月初旬に or 来月初旬.
桜は四月初旬に満開になる予定です。
The cherry blossoms are expected to be in full bloom in early April.
新しい店は来月初旬
The new shop will open in the first ten days of next month.