>Hi,
>
>In my application Week Number is a very important concept. And from the beginning of the app (many years ago), in my code, I always considered week by Monday of the week. That is, a week should have Monday.
>Next year, 2019, is a little challenging. The 1st of January falls on Tuesday. So, my app will consider the first week to be Week of January 7th (first Monday). But I know that many customers will not accept it (or at least will not understand it). And they will still think of the week of December 31 of 2018 as the first week of 2019.
>I can change it in code (I think) and have the program re-calculate the 1st week and then always, when looking for a week #, to take into account that the first week was actually not the full week.
>
>Do you think that I should just stick to my initial design and have the week of January 7th, 2019 to be the first week of the year? Or re-calculate (kind of late but I can do it).

Just do the same as you did in 2013.

Changing the habits of hundreds of users is harder than changing code. But then if week number is so important in your app, and it lives for so many years, then why change the definition of such an important concept, just because "customers will not accept it". Same folks who didn't accept that this century began on 1/1/2001, not 1/1/2000?

What did they do in 2013? It began on a tuesday.