The window is narrow and specific, which is itself a warning sign: a pattern defined by exactly seven sessions has the flavour of something found by searching rather than predicted in advance.
Suggested causes include light holiday volume, year-end bonus investment, tax-related positioning and general optimism. All are plausible and none is demonstrated.
Some analysts treat the absence of the rally as a bearish omen for the following year, which stretches a small sample very far. With roughly one observation per year, even a century of data gives a modest number of trials, so confidence intervals around any claim are wide.
Related: seasonality, january-effect, turn-of-the-month-effect, sample-size, sell-in-may