There are 52 weeks in a year, so there will be 52 sundays, no two questions about it.
However, 52×7=364 so that still leaves a day for a normal year, and two days for a leap year.
Now, let us look at the combinations of the two days possible.
(Sunday, Monday), ( Monday, Tuesday), (Tuesday, Wednesday), (Wednesday, Thursday), (Thursday, Firday), (Friday, Saturaday), (Saturaday, Sunday)
Now, to get exactly 52 sundays, none of the last two days should be a Sunday. As you can see from the combinations listed above, that is possible in 5 out of 7 cases. So the Probability would be 57