Massachusetts law provides 6 reasons for a landlord or his agents to enter a leased apartment. They are:
To show it to a prospective renter or purchaser, mortgage company, insurance company, etc.
To inspect the apartment
To make repairs
In accordance with a court order
If the premises appear to have been abandoned
To inspect the premises within the last 30 days of the tenancy or to determine the amount of damage to be deducted from the security deposit after notice to terminate has been given.
Common courtesy would dictate advanced notice, however unless it is specifically stated in your lease, advance notice is not required. This means that they can enter without notice and even if it is inconvenient for the reasons stated above.