Continuing the chatter in my earlier discussion of why January 1 is being reported as the last week of the previous year.
Commenter Smithers notes a few things, which I will use as my springboard.
Seriously though, USA, why can't you let a week that's in two years at once just be one week? Also FirstDayOfWeek=Sunday?? It's called the week*end*, it comes at the *end* of the week.
As I noted in my inline comment: Like a rope or stick or soccer pitch, a week has two ends. There's the front end (Sunday) and the back end (Saturday). One interpretation of the word "end" is is "not the beginning"; another is "extremity".
The reality is that the term "end" in "weekend" doesn't actually mean "end". The term "weekend" refers to any day that is not a day of work. Hence phrases like "long weekend".
And then there are the religious-historical reasons why Saturday is the seventh day of the week, which in turn makes Sunday the first day of the week.
But I'm really here to talk about week numbers.
Basically, in the United States, we don't use them. So go ahead, Europe, use your fancy week numbering schemes, with the rule of four or the rule of Thursday or the rule of St. Benedict. (I may have made up that last one.)
Since the United States doesn't use week numbers to any significant extent, the definitions are chosen to align with how people talk in casual conversation: "Week 1" is the first week of the year, and it begins on January 1. The weeks are then numbered consecutively, with each subsequent week starting in on Sunday.
This numbering scheme means that most of the time, Week 1 is a partial week, and so is Week 53.
I should emphasize that you will not see a calendar in the United States that looks like this, because nobody cares about week numbers.
This simple numbering scheme aligns with how people talk about weeks. "It'll be done in the last week of 2015" means "It'll be done some time between December 27 and December 31 of 2015." Nobody expects "the last week of 2015" to extend into 2016.
Similarly, if somebody says, "It'll be ready in the first week of 2016," then if it is ready on December 27, 2015, it will be considered to have completed ahead of schedule. Nobody expects "the first week of 2016" to extend into 2015.
In the United States, the conventional calendar milestones are not weeks; they are months. Things are scheduled for "the first week of November" or "the middle of October" or "the end of April." And the definition of "the first week of the month" is "Starting on the first day of the month, and extending until the first Saturday of the month." Similarly, "the last week of the month" is "Starting on the last Sunday of the month, and extending until the last day of the month."
I remember when I visited Sweden, I saw a notice that said something like "Opening in Week 23!" That meant nothing to me. I didn't know what week number were were in currently, so I couldn't calculate how many more weeks before the building opened. I also had no sense of whether Week 23 was springtime or summertime or autumn. The weeks may as well have been assigned colors or shapes.