Project Euler #19: Counting Sundays in the 20th century using Pandas
It is good to use a library rather than re-inventing everything yourself. Just be sure to avoid explicit looping in Python:

sum(date.day == 1 and date.weekday() == 6 for date in rng)


The above sums the number of times that date.day == 1 and date.weekday() == 6 automatically, with no loops of counters, It should also be more efficient (sum is implemented in C)