Did you know you can pass a day of the week into Ruby's Date.parse function? For example you could do something like this;
friday = Date.parse('Friday') p friday => #<Date: 2013-07-12 ((2456486j,0s,0n),+0s,2299161j)>
This is pretty useful, it seems to return the date closest to the day you enter, and of course this will be based on your server's date and time settings.
If you wanted to work out the date at the start of the current week and your week starts on Sunday, you could do something like;
sunday = Date.parse('Sunday') if sunday > Date.today sunday -= 7 end
This is a super quick way of playing with dates without needing to rely on any gems.