Hi Filipe Hemsworth,
I think these two pieces of information could be helpful to you:
1. DATETRUNC has an optional [start_of_week] parameter. You can set it to whatever you want so that your function has the following format: DATETRUNC(date_part, date, [start_of_week]).
2. If you did not specify the start of week, Tableau will use the data source to determine the start of week. You can learn more here: Date Properties
I've created a blog post about time period comparison using a similar approach. You can find my solution, together with the workbook, there.
Hope this helps!
In-case others have had this issue, here is the solution I ended up using. A bit of a rough work-around, but it should work for the next few years until the number of days between dates year on year change from 2.
DATETRUNC([Period], [Date]) =
DATETRUNC([Period], DATEADD('year', -1,[Date Selection]))
DATETRUNC('day', [Date]) <=
DATETRUNC('day', DATEADD('year', -1,[Date Selection])+2),
DATETRUNC('day', DATEADD('year', -1,[Date Selection])))