y Year Year 1996; 96
Y Week year Year 2009; 09 "
I have tested those two keywords for creating patterns, and they produce the same result.
What do they mean by Week year in that context?
A week year is a year where all the weeks in the year are whole weeks. This is specified by some standard (which I don't remember at the moment). Basically, this guarantees that a program working on a week's data will not transition between years. Unfortunately, this also means that the beginning of the year may not start on the first of January. What year a particular day belongs in depends on these rules, and of course, there are days where the year and the week year are different.