Why so?
Well in date form it contains three of our often praised power number 9s (one transposed alpha to numeric as in Monday adding up to 9 see below, one immediate as in 9th , one resolved as a single digit wherein 2016 adds up to 9) along with the fact that our much vaunted number of balance 5 sits perfectly in the middle. Obviously the power numeral is a thing of both light and dark, good and evil and so on, but on this day of days equilibrium shall reign, a definite day for good.
MONDAY
13-15-14-4-1-25
4+6+5+4+1+7 = 27
2+7 = 9
and Mid-Winter, hence it is known in Latin as a biflora species.
Legend has it that the original thorn was brought to Britain and planted
by Joseph of Arimathea as a symbol of the arrival of Christianity.
Glastonbury Tor is the hill in the background.