Introduction:
The Gregorian calendar has a leap year after every four years, except for years that are divisible by 100 but not by 400. The year 2000 was a leap year, but 1700, 1800, and 1900 were not.

Calculating the Probability:
To calculate the probability of a leap year having 53 Sundays, we need to follow these steps:

1. Find the number of days in a leap year. A leap year has 366 days.
2. Divide the number of days by 7 to find the number of weeks. 366 / 7 = 52.2857
3. Since we cannot have a fraction of a Sunday, we need to round down to 52 full weeks.
4. This means that a leap year will have 52 Sundays and one additional day.
5. The additional day can be any day of the week, so there is a 1/7 chance that it will be a Sunday.
6. Therefore, the probability of a leap year having 53 Sundays is 1/7 or 0.142857.

Explanation:
The probability of a leap year having 53 Sundays is relatively low because there are only seven possible days that the additional day could fall on. If we think about it another way, a leap year will have 52 full weeks, so it is more likely that there will be 52 Sundays and one day that falls on a different day of the week.

Conclusion:
In conclusion, the probability of a leap year having 53 Sundays is 1/7 or 0.142857. This means that it is relatively uncommon for a leap year to have 53 Sundays, but it is still possible.