I recently came across a useful site which shows week numbers for calendar years -for example what date is "week 5 of 2014"?
This is handy when you estimate you have, say 20 weeks' work to do, know when you can start and want to estimate when you will finished. Or, coversely, you have another 2 of those parameters and want to work out the third (e.g. 20 weeks' work and a deadline; when ought you to start?)
A table of week number can be handier than taking a calendar & counting out the weeks with your finger. But however you do it, be careful not to make a fencepost error !
You can start work at the beginning of week 2 of 2014 (Monday January 6th). You have 20 weeks' of work to do; when will you finish? The intuitive answer - "in week 22 (calculated by 2+20)" is wrong - or at least a bit fuzzy. Assuming it really is 20 weeks' work, you will finish at the end of week 21 (May 25th). Or at the start of week 22, (Monday May 26th) if you prefer - you should not need to be working during week 22, unless there was more work or slower progress than anticipated. That's because you did some work in week 1 - and so you are counting the actual weeks, (the "fence panels" not the "fence posts") .
Luckily here any fencepost error is likely to be in your favour (your mistake leads you to think you need more time). Much more distressing to discover you have a week less than you thought!