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Answer:
- ((ordinal date) +3) mod 7 = weekday number
- (IFC date) mod 7 = weekday number
- no rule is possible
Explanation:
a) Ordinal Calendar
The ordinal calendar numbers days of the year sequentially, so any given date is specified only by its day number: 1 to 366. In 2020, Jan 1 was a Wednesday, so the rule needs to be ...
day of week = ((ordinal date) +3) mod 7
This will give a result from 0 to 6. Those numbers map to days of the week as follows:
0: Saturday; 1: Sunday; 2: Monday; 3: Tuesday; 4: Wednesday; 5: Thursday; 6: Friday
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b) International Fixed Calendar
The international fixed calendar has 13 months of 28 days. The first day of the year is always Sunday, so the day of the week is ...
day of week = (International Fixed Calendar date) mod 7
In 2020, there are two "leap days" that have day number 29. Days numbered 29 have no associated day of the week.
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c) There is no rule that maps weekday to day of the year. (Each weekday is repeated many times (52 or 53) through the year, so there can be no functional relationship.)