The isoweek module provide the class Week. Instances represent specific weeks spanning Monday to Sunday.
There are 52 or 53 numbered weeks in a year. Week 1 is defined to be the first week with 4 or more days in January.
It’s called isoweek because this is the week definition of ISO 8601.
This standard also define a notation for identifying weeks; yyyyWww (where the “W” is a literal).
An example is “2011W08” which denotes the 8th week of year 2011.
Week instances stringify to this form.