s is indexed by daily dates. Return the mean of the values whose date falls in February of a leap year.

Sample data: s:

2020-01-15    0.0
2020-01-16    1.0
2020-01-17    2.0
2020-01-18    3.0
2020-01-19    4.0
2020-01-20    5.0
Freq: D
... (60 rows total)