just after Obon; the period immediately after Obon
Refers to the time right after the Obon festival (mid-August in most regions), when businesses and schools resume normal activity.
See also: お盆
On the Monday just after Obon, the roads are very crowded.
Many schools start the new term right after Obon.
Compound of 盆 (ぼん, 'Obon festival') and 明け (あけ, 'end' or 'after'). The exact historical derivation is straightforward.