...the answer my surprise you.  From Mass Legal Help: "You must pay the rent in advance, on or before the date you and the landlord agreed rent is due. Often the agreed rent date is the first of the month." From the Tenants Rights page on the official Mass.gov website: "...the landlord can begin the eviction process immediately, even if the rent is only one day overdue." So the answer is: on day two....

The lead laws in Massachusetts are changing and they will effect residential landlords.  As your property manager it is my job to stay on top of these things.  The following post is mostly composed of excerpts from official sources (mass.gov website, legal blogs, licensed deleaders, attorneys, etc.), and all have been cited so you can see them in their entirety. Lets recap the most important aspects...