How is it to do a bike tour in Boston in December, would it be too cold? And what about the Freedom Trail?

Too cold ... not really (I've ridden at temperatures down to -35C). But it would certainly be "cool" weather requiring fall/winter gear (gloves, hat/ear covers, tights, cycling jacket, maybe booties). If you were unlucky there could be snow ... which at best would make for messy riding and at worst make unplowed trails impassable.

Really, at temperatures much under 40-45F bikes get harder to crank (oils get stiffer), and more than a couple of hours can get cold unless you are correctly dressed and avoid sweat accumulation.

Is a tour possible ... sure. Is it as much fun as warmer weather ... no.

The Freedom Trail is mixed surface (brick, stone and asphalt), which will be miserable if there is ice or snow. Also, Boston is a rolling type of topography and the Freedom Trail requires many road crossings, all of which would stink in ice and snow. The Trail is only something like 3.5 miles so it would be a much better run.

I was just in Boston last week and walked the Freedom Trail with my son. We saw lots of cyclists in the streets and none of them were smiling. The traffic congestion is some of the worst I've ever seen, making cycling in that city quite a challenge and quite dangerous.

Have you ever thought about biking the Finger Lakes in New York?

could be very cold avg dec temp is in the 40's in boston.