Since the resorts fill or come close to it almost year round, busy is defined not by whether rooms eventually fill but how fast they fill once the 11 month reservation window opens and then what is still avaialble at 7 months out when members can reserve at resorts other than the one they own. DVC's low to moderate season at WDW begins the Monday after marathon weekend in January and ends just before the last weekend in Sep. During that time, even during holidays, you can usually find near park resort rooms such as BWV pool/garden view, BLT lake view, VWL, Poly, and VGF (except for studios) open at 7 months out and BCV is also avaialble much of the time other than holidays and special weekends.
The high season (and it is a very significant difference from the rest of the year) is late Sep through marathon weekend in Jan. During that time, getting anything at near park resorts becomes difficult at 7 months out and even 1BRs (usually the last to go) can often disappear. That last quarter has certain extremely high demand times when there is a real risk of not getting certain room categories even at 11 months out at near park resorts such as VGF studios, BLT standard view, BWV standard and boardwalk view (Poly is still an unknown), and even AKV and OKW have some risk of disappearing for some days before 7 months out: (a) first week of December, the highest demand week of the year; (b) Christmas week and second week of December which are close to tied for second for highest demand weeks; (c) Thanksgiving Tuesday through Friday; (d) four weekends (and by weekends I mean Thurs through Sun) -- marathon weekend in January, Columbus Day weekend, the weekend after Columbus Day, and the weekednd in the first part of Nov when the Wine & Dine race occurs. The two highest demand nights of the year are NYE and the Friday of that Wine & Dine race weekend in Nov when even SSR can disappear before 7 months out.