Concept:
- Any date that is a multiple of 7 days away from a known date falls on the exact same day of the week.
- This lets us jump forward from the given date in steps of 7 days to land on a date very close to the target date, so only a small remainder needs to be counted instead of the entire gap.
Step 1: Note the given date and the target date
21 July is a Wednesday. The target date is 25 December of the same year.
Step 2: Move forward in jumps of 7 days from 21 July
Adding 7 days repeatedly keeps the day fixed at Wednesday, so 28 July, 4 Aug, 11 Aug, 18 Aug, 25 Aug, and so on up to 22 December are all Wednesdays. This can be checked in one shot: from 21 July to 22 December is $10 + 31 + 30 + 31 + 30 + 22 = 154$ days, and $154 = 22 \times 7$, a whole number of weeks, so 22 December is also a Wednesday.
Step 3: Count the small remainder to 25 December
25 December is only $25 - 22 = 3$ days after 22 December.
Wednesday plus 1 day = Thursday
Thursday plus 1 day = Friday
Friday plus 1 day = Saturday
Final Answer: Christmas (25 December) falls on Saturday.