Given any date, find the first date of the week in Python

Given a date like "9/23/2016", find the first date of the week. The below code will do so

from System import *
from System.Globalization import *

class Program(object):
	def Main(args):
		initialDate = Convert.ToDateTime("9/23/2016")
		firstDayOfweek = initialDate.AddDays(CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek - initialDate.DayOfWeek)
		Console.ReadKey()

	Main = staticmethod(Main)