1. Traditionally, they have filmed the Disneyland portions of the Christmas Day special during the first two weekends of November, with them taping the parade portion on the 1st weekend (since they use Christmas Tree Point for the camera/host duties.)
After the parade portion has been recorded, the tree can go up, and the garland is strung across Main Street. Sometimes they have done it all, on the first weekend of November and the next couple of days. I think a lot of that is determined based on when DL's TV Host is available for shooting.
Since several outside groups are involved in the Parade portion tapping, news as to the exacts dates for that will like leak out ahead of time, to give you advance notice. They also often set up alternative dates/locations in case of inclimate weather. A year or two ago, (I believe it was for Christina Aguilara) the weather outside wasn't cooperating, so they taped her portion inside the Grand Californian.
2. As for who performs, that information is not usually released until the day of the performance. Sometimes word might leak out the day before. But they have been very tight lipped on this info.
Oftentimes, the performers do their songs the same weekend as the parade. Usually a separate stage area is set up for the performance, whether it's in front of the castle and/or around the main hub in DL. Sometimes it's done right in front of Christmas Tree Point, between different parts of the parade. When Princess and the Frog came out, they did some recording in New Orleans Square. Last year, during the first week of December, they filmed one of the performers on a Special Stage right in the middle of Radiator Springs, on the street between Flo's and Lizzie's. So, not everything is necessarily taped the first part of November.
Besides the special stages/camera set ups, the next biggest clue for the filming is some standing signs that DL will place in the areas where the filming is going on.