Marijuana becomes legal in Massachusetts on Thursday. Or rather, it becomes legal to consume and to buy, grow, and possess in limited quantities for personal use. It still will not be legal to sell marijuana without a medical caregivers license.
Just like Maine, Massachusetts has to draft its laws and regulations for retail sale. Retail sale won't be legal until at least January 2018.
Of course, marijuana is still illegal under federal law. Please don't transport across state lines!