Winter Solstice Festival (冬至）is a day when Hong Kong local people gather together. It is 22nd December in 2019.
Traditionally Chinese people would gather together with their families for a nice deal and exchange presents, and would eat Tangyuan as dessert after dinner to symbolise family togetherness.
Moreover, colleagues would usually gather together for lunch to celebrate.
There is a saying "Winter Solstice is more important than Lunar New Year" which reflects how important the day it is. Therefore many restaurants especially Chinese ones will probably be fully booked that day.
HONGKONGUIDE suggests visitors should make reservations for the restaurants rather than walk-in.
Photo source: Liberty Times Net