Time of year doesn't matter, but time of day does.
Early in the a.m. (more than 1 hour before park opening), all buses are shared.
From 1 hour before park opening until 11 a.m., most AS buses serve just one resort; a few may share.
After 11 a.m., most MK buses still serve one resort; each of the other parks' buses share the 3 resorts.
DTD buses always share.
When the buses are shared, most will go in this order: Sports, Music, Movies. Occasionally, a bus will be sent in reverse order, usually after one or more buses have filled up before reaching Movies.
Returning from the parks, MK has separate buses. During the peak return times (right after fireworks or Fantasmic!), Epcot and sometimes DHS will have separate buses. The rest of the time, and from DAK, buses will share Sports, Music, and Movies.
Everything is subject to change. Enhancements are in the works for DHS buses; I don't know whether that will include separate AS buses at more times.