New Year's Week; matsu-no-uchi period
Refers to the first days of the New Year when traditional decorations (kadomatsu) are displayed. The exact dates vary by region: typically January 1–7 in eastern Japan (Kantō) and January 1–15 in western Japan (Kansai).
I must write replies to New Year's cards before the matsu-no-uchi period ends.
In Kansai, matsu-no-uchi lasts until the 15th.