I've sorted out this issue and updated the formula:
IF DATETRUNC('week', [Day]-1)+1 <= TODAY()
AND DATEDIFF('week',DATETRUNC('week', [Day]-1)+1, TODAY())= 0 THEN [Sales] END
In case it helps, I developed a way to have the week to date for all the weeks, not only the current one.
The formula is:
if DATEPART('weekday', [date]) <= DATEPART('weekday', today()) then [Sales] END
This formula provides the week-to-date for all the weeks, so in order to compare just need to drop Sales into columns and filter previous 2 weeks, if you just want to create this week vs previous.
I hope it helps
P.S.: I developed the same but for non-us as weeks starts on Monday, if someone needs it please just send me a message