SOS - How to get the number of a week

I give an SOS to the world ... How can I get the fisrt and last days for a given week : Week = 40 of 2000 ===> first = monday 10/2/2000 last Sunday 10/8/2000 It's more difficult when the week is beetween december and january or when there are 53 weeks a year! 12/31/2001 is first day of week 1 - 2002 12/30/2002 is first day of week 1 - 2003 Some years have 53 weeks (when ?). I hope someone can help me !!!!