The stretch of days between Christmas and New Year’s Day, during which altered routines and relaxed schedules result in a loss of awareness of time and dates.

From Merry (associated with Christmas) + –neum (modelled on ‘perineum’, indicating an in-between space).

Source: https://www.dailymail.co.uk/news/article-9084551/Countdowns-Dictionary-Corner-wordsmith-SUSIE-DENT-reveals-12-festive-linguistic-gems.html.