first third of a month; first ten days of a month
Refers roughly to days 1 through 10 of a month. Common in written and spoken scheduling expressions such as 4月上旬 and 来月上旬.
A new shop will open in early April.
The results will be announced in the first ten days of next month.
I'm planning to go on a trip in the early part of next month.
The meeting will be held in the early part of April.