Calendars repeat when the number of days between years is a multiple of 7 and leap year status matches. From 2017 to 2023 is 6 years with 1 leap year (2020), totaling 2191 days (2191/7 = 313 weeks exactly). 2017 and 2023 are both non-leap years starting on Sunday, so their calendars are identical. 2024 is a leap year, 2027 is 7 years later, and 2028 is a leap year, so their calendars differ.