The first week of containing dates in January starts on a Friday. If your week starts on Sunday, then your first week would only have two days (Jan 1 and Jan 2).
What you want to happen is you want Domo to 'just know' that Jan 1 and Jan 2 belong to the last week of December... which is business logic, not how a calendar actually works.
Short version of the story is you should implement a date dimension where you can control how business logic is applied to your dates (in this context, how weeks are counted when a month or year starts in the middle of the week).
Alternatively @GrantSmith recommends calculating the WeekNumber of the Start of the Week.