Who sets the schedule in Congressional roll call votes? So who or which group decides when Congress votes on what issues and how flexible is this process? Is there some scope such that the votes on certain bills can be preponed or postponed depending on general circumstances?
The leaders of the two houses control the schedule and can interrupt it or hurry it in an emergency. This is one reason why the parties find it important to control each chamber.
Note than in the "normal order", bills are scheduled ahead of time. They follow a process that includes a time in committee, time for debate on the floor, and a vote. In the House, this process is normally set by the Rules Committee.
In the Senate, the Majority Leader can call for a cloture vote--that's essentially what the sixty vote requirement for ending a filibuster is. Although in an emergency, they would be more likely to ask the current speaker to cede control of the floor voluntarily. These procedures are controlled by the Standing Rules.
If you watch the news, you'll see examples of the two chambers postponing votes for various reasons. The most common is that they want more time to line up enough votes.