I hate it when people use week numbers, like "week 34". Week numbers are mostly not printed on any calendar or schedule. So; here is how to convert a date to a week number and a weeknumber to a date.
This is an easy one, because the man page of date simply explains:
%U - Displays week of the year(Sunday as the first day of the week) as a decimal number[00 - 53] . All days in a new year preceding the first Sunday are considered to be in week 0.
$ date +'%U'
A little harder, that why you don't have to figure it out yourself, just copy-paste and replace to meet your requirements:
$ date +'%U' 2009-10-2
This is not an easy one and requires a very nasty trick. To get a day in week 23 for example, use this command:
$ date --date="$(((23-$(date +'%U'))*7)) days"
Mon Jun 8 12:11:40 WEDT 2009