One other person (but only one that I recall) recently complained of a similar situation, I do not know the cause.
You can try setting changing the "Week Start Day" in the Calendar Settings administration section to see if that makes a difference.
Failing that, a temporary work around might be to rename the Weekdays in that same section although that should only be a last resort as I have no idea what other affects it might have.
Let us know if you find the solution (or the cause).
Dan O.
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