I maintain a weekly list of current tasks in text files for work and personal projects. I used the week of the month for filename previously (such as 2015-09-01), before I realised I could employ another unique value: the week number of the year.

But how does one find it? It’s not on most calendars, and I certainly don't track them in my real life. Fortunately, *nix date to the rescue:

$ date +"%Y"
Or randomly from my university Oracle notes, if that’s your thing:
SELECT to_char(sysdate, 'IW') FROM DUAL 

For those curious, we're in week 36.