In the Boston area, this can be a tough call. It can be 65 degrees, and feel like's it much colder. Or.. It can be 65 degrees, and feel much warmer. The wind, relative humidity,
etc. can play parts. Wind from the east: Long tights and a jacket. Carry ear protection and warm gloves. You will not regret this. Wind from the west: Even tougher. A west wind
can actually be quite warm, even though the west wind is stronger. i.e. Plastic-trash-cans-in-the-street, lots of airborne pieces of trash.

What it takes: Experience in your given geographic area. But, I basically consider it "winter" until the temps go up to the 70s and stay there.