• Japanese people use 10-day periods called jun (旬). Each month is divided roughly into three 10-day periods:
  • The first (from the 1st to the 10th) is jōjun (上旬, upper jun)
  • The second (from the 11th to the 20th), chūjun (中旬, middle jun)
  • The last (from the 21st to the end of the month), gejun (下旬, lower jun).
• These are frequently used to indicate approximate times, for example, "the temperatures are typical of the jōjun of April"; "a vote on a bill is expected during the gejun of this month."

from Wikipedia