In a leap year, there are 366 days. Out of these 364 days make 52 weeks and hence 52 saturday. Remaining 2 days can be: Sun-Mon, Tue-Wed, Wed-Thu, Fri-Sat, Sat-Sun.
Total number of possible outcomes = 7
No of saturday outcomes = 2 (Fri-Sat, Sat-Sun)
Then probability of 53 saturdays = 2/7