There are exactly 20,871 weeks in 400 Gregorian years, so 30 January 1622 was a Sunday just as was 30 January 2022. In a Gregorian mean year, there are 365.2425 days, and thus exactly 52 + 71⁄ 400 or 52.1775 weeks (unlike the Julian year of 365.25 days or 52 + 5⁄ 28 ≈ 52.1786 weeks, which cannot be represented by a finite decimal expansion).