noun, adverb
during; for the duration of
Used to indicate that something happens or is valid throughout a specified period. Often follows a time expression (e.g., 夏休み期間中, キャンペーン期間中).
夏休み期間中は、図書館の開館時間が短くなります。
During the summer vacation period, the library's opening hours will be shortened.
キャンペーン期間中にご注文いただくと、送料が無料になります。
If you order during the campaign period, shipping is free.
「中」 attached to a time noun (e.g., 今月中) also means 'during', but 「期間中」 explicitly includes the word 'period', making it suitable when the period itself is the topic.
Compound of 期間 (period) + 中 (during). A straightforward Sino-Japanese formation.