I agree with Grannie G. Sadly there is no real solution. When someone with dementia loses their time sense, there's not much you can do. There is every chance that your wife:
Will forget to look at the calendar
Will look at it and then immediate forget what she has read
Will read the calendar and not believe it, being convinced that what her time sense is telling her is right
Will eventually lose the ability to comprehend what the calendar is telling her, and even the ability to read it
We went through this exact same problem with my father and all the "solutions" we came up with like calendars, reminder notes and so forth failed or at best worked for a month or som because of the things above. Dad would just forget to look at the calendar or wouldn;t believe what it said, being convinced he was right and the world was wrong (a recurring dementia theme). At worst it lead to rows and claims that were were using "american time" and such.
The most difficult phase was when he thought he knew what the time and day were but was wildy wrong (leading to demands for breakfast "before going to work" at midnight, etc). Eventually he totally lost all ability with time which was actually easier to manage even though it lead to far more repetitions of "what time is it" and "what day is it". Even that eventually passed because he became completely detached and no longer even had any concept of time or day and became more complaint - in that regard, anyway.