We just did the northern triangle in late December (myself, wife, teenage daughter). We added on 2 days at Ranthambhore National Park, which was a lot of fun. (Tiger safaris, etc).

We thought the weather was excellent (we're from Los Angeles). Weather was in the mid- and upper-70's the whole time (F). No rain. There was a little bit of fog on some mornings, but generally clear skies by lunch. Never had mosquito problems.