Not sure I understand the question, but it seems like you are suggesting the employer is changing the rules in an effort to avoid paying overtime for just one week.  I say that because, eventually, the employer will still have to pay overtime on weeks with five work days when an employee works an extra day.

I don't know the answer to your question.  I would guess that an employer can establish a different work week as long as it does not violate the law in the process.  For example, I guess you would argue that by shifting the work week the way it did, you employer skirted Massachusetts General Laws Chapter 151, Section 1A:

Section 1A. Except as otherwise provided in this section, no employer in the commonwealth shall employ any of his employees in an occupation, as defined in section two, for a work week longer than forty hours, unless such employee receives compensation for his employment in excess of forty hours at a rate not less than one and one half times the regular rate at which he is employed.

When in doubt about matters that are not "big" enough to warrant the assistance of an attorney, I always recommend that readers contact The Attorney General's Fair Labor Hotline at 617-727-3465.  Sorry I can't be of more help.  Maybe an attorney with employment law experience will offer an opinion.  For more information or to post a questions, visit our Massachusetts Employment Law Discussion Forum.