The probability that a leap year will have 53 sundays is

The number of days in a leap year is 366

Leap year have 52 weeks and 2 more days

To have 53 Sundays in a leap year, one of the extra two days must be Sunday

The possibilities are $$\begin{gathered} S=\left\{\left(Sun,Mon\right),\left(Mon,Tue\right),\left(Tue, Wed\right), \\ \left(Wed,Thu\right),\left(Thu,Fri\right),\left(Fri,sat\right),\left(Sat,Sun\right)\right\} \end{gathered}$$

hence, $n\left(S\right)=7$

The number of favorable cases to the event of getting one of two days must be Sunday is $2: \left\{\left(Sun,Mon\right),\left(Sat,Sun\right)\right\}$

Therefore,The probability that a leap year will have 53 Sundays is $\boxed{\frac{2}{7}}$