土曜日に予定がない
I don't think this に marks time because 予定がない is not something that happens. My closest guess is that it denotes 評価.
ない being the negative form of the verb ある, your sentence has the same basic structure as a sentence like this.
月曜日に試験がある。
This に clearly indicates a specific time when something happens. The difference, of course, is that the subject in your sentence is not an action or event, but a plan. But then again, this plan is one for doing something. People will understand this 予定 as すること and think of some action or event when they hear someone say 予定がある or 予定がない. This must be making に acceptable enough.
While totally acceptable in everyday conversation, the following polite versions still don’t quite sound natural to me as standalone sentences.
土曜日に予定があります。
土曜日に予定がありません。
The following are totally unacceptable as スケジュール cannot be understood as the same as すること.
x 土曜日にスケジュールがあります。
x 土曜日にスケジュールがありません。
In this case「土曜日」(Saturday) is a time indication, so this usage of に is temporal. The word「予定」means schedule or plan, but it's also used to mean plans (i.e. things to do). Your sentence therefore translates to:
I have no plans on Saturday.