有冇辦法唔用蔡勒公式都可以極快計到星期幾?

有辦法!
Year:
subtract 1900 to give 99, divide by 4 (=24.75, use 24) and add to 99 to give 123.
divide by 7 and keep remainder (=4).
Month:
Memorize the sequence 033614625035 for the 12 months. For 9月, the number is 5, add this to the (4) above. (4+5=9)
Add the day number 9日 to 9 (9+9=18).
Divide the sum by 7 to give a remainder of 4 (=星期 4).
This method works from 1900 up to 2999.
If the day is before 2月28日, then subtract 1.
With a little practice, you can get it in 10 seconds, I've done it.