Step 1: A non-leap year has 365 days, and since a week contains 7 days, we divide 365 by 7:
\[
365 \div 7 = 52 \text{ weeks with 1 extra day}
\]
This means the year consists of 52 complete weeks and 1 extra day.
Step 2: Since there are 52 complete weeks, there are 52 Sundays in the non-leap year. The extra day can be any one of the 7 days of the week, including Sunday.
Step 3: To have 53 Sundays in the year, the extra day must be a Sunday. Since the extra day is equally likely to be any of the 7 days, the probability that it is a Sunday is:
\[
\frac{1}{7}
\]
Thus, the probability that a non-leap year contains 53 Sundays is \( \frac{1}{7} \).