This is not the place for legal advice, unfortunately.
Only lawyers can give legal advice.
You might be able to get your answer from a tenants' rights lawyer.
In general, a lease in MA usually contains a provision for reasonable notice.
Reasonable notice probably can be defined by a lawyer.
Unless they are suspecting you of something inappropriate, they should respect your privacy - in my opinion.
However, if you are not making the showing of your unit "reasonable" once they have given you "reasonable notice", they might fell that an intrusion is acceptable.
That does not make barging in any more correct than a tenant making it hard to access a unit.
Keep in mind, the landlord needs to keep the apartment rented.
This is their business, and the unit is part of their inventory.
If a tenant is affecting a landlords' ability to make a living, they might consider that unreasonable.
Keep in mind that a good reference from a past landlord is valuable.
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