Today is winter solstice (冬至), one of the important solar terms (节气) in Chinese lunar calendar. In this day, it is the shortest day of the year for north hemisphere. Chinese believe that when we reached to the darkest end, it will be the beginning of the brighter side. Therefore we celebrate the longest night of the year and after this day, the sun will move towards north and the day will getting longer and longer from day to day.
Due to winter solstice is based on the sun instead of moon; therefore it is almost fixed every year between 21st - 23rd December. To calculate the exact day for winter solstice, we can use the following formula:
Winter solstice day = Y * D + C – L
Y is the last two digit of the year,
D is the a constant, which is 0.2422,
L is the number of lead year
And for C is another constant, for 21st century, C = 21.94 and for 20th century, C = 22.60.
For example, when is the winter solstice fell on 2009?
The answer is (9 * 0.2422) + 21.94 – (9 / 4) = 21.87. Therefore it is fell on 22nd Dec. 2009.
Based on the calculation, we will going to celebrate winter solstice on 22nd Dec. until year 2056 and start from year 2057 we will going to celebrate winter solstice on 21st Dec. Not sure I'm still there or not :)
Ok, now forget about this math and back to normal life.
Winter solstice is very important in Chinese culture and it is more important than Chinese New Year because it’s the beginning of the "bright side". Therefore, this day family members gather together and have a nice and warm dinner and follow by sweet dumpling (汤圆) as symbolic of reunion.
Once you ate the sweet dumpling, it mean, you are older another year :)
So, happy winter solstice and happy grow up.