Because most city apartment leases start September 1st, the streets become chalk full of double-parked cars, vans and trucks starting August 31st as tenants start the arduous task of moving from one apartment to the next. The result of having literally tens of thousands of people all moving on the same day is that the city sidewalks become littered with unwanted items. This ‘littering’ of every piece of furniture and furnishing imaginable is what is referred to as an Allston Christmas. Although this city-wide move impacts nearly all of Boston’s neighbohoods, nowhere is the experience more intense than Allston, although many neighborhoods will likely experience their own form of an Allston Xmas.
In case you think I’m making this up, Urban Dictionary added this term in 2008 here and one only needs to type in the phrase in Google and select images to see what the streets of Boston will look like starting August 31st.