应该这样做：
from datetime import datetime
def is_third_friday(s):
d = datetime.strptime(s, '%b %d, %Y')
return d.weekday() == 4 and 15 <= d.day <= 21
测试：
print is_third_friday('Jan 18, 2013') # True
print is_third_friday('Feb 22, 2013') # False
print is_third_friday('Jun 21, 2013') # True
print is_third_friday('Sep 20, 2013') # True
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