I can compile a list if needed and post, but I noticed this over 50 years ago, My Father, My Brother and Myself our Birthdays fall on the same day of the week every year. Even Leap years, that does not change this.
Years ago before computers, I went to the Local Library, did research and Found the "Roman Calendar" was the most accurate to use. I compiled on Paper via Pencil a Calendar based on the "Roman" principle, I made a template and as I said, Our birthdays fall on the "same day" every year. All three of us. How can this be.
Now we have found that 6 other family members have this on our Birthdays being the same day. This makes a total of 9 of us.
This all has to do with a number of "7" as a constant. I am no Mathematician by no means.
Can someone who knows their stuff respond. I mean I am 65 and the days have always been the same refering to a same weekday for our birthdays every year.
If all the birthdays in question are in months after February, then the difference between any two of the birthdays is the same number of days every year, even in leap years. If this difference is a multiple of 7, then the birthdays fall on the same day of the week.
A related mathematical topic is modular arithmetic. Converting between dates and days-of-the-week is closely related to working with integers mod 7.