The Sprint is a fixed-length event of one month or less, and every other event happens inside it. A new Sprint begins immediately after the previous one concludes, which is what gives Scrum its cadence.
What is fixed and what is not
The length of a Sprint is fixed, and so is the quality of what comes out of it, because the Definition of Done sets a standard that does not move to make a deadline reachable. Scope is the part that flexes. As the Developers learn what the work actually involves, they and the Product Owner may clarify and renegotiate it, so long as the Sprint Goal itself is not put at risk.
That combination is the whole bargain of a Sprint. You get a fixed date and a fixed standard, and in exchange the amount of work delivered is allowed to change as understanding improves.
The three constraints on a Sprint in progress
Three rules hold for the length of a Sprint, and they follow from the bargain above. No change may be made that would endanger the Sprint Goal, since the goal is the commitment the Sprint is being run to meet. Quality does not decrease, since lowering the Definition of Done would hide unfinished work rather than remove it. And the Product Backlog is refined as needed throughout, because refinement is ongoing work rather than a scheduled event, and stopping it would leave the next Sprint Planning with nothing ready to select.
Refinement is worth noticing here because it is the one of the three that describes something a team should actively be doing rather than something it must avoid.
Shorter Sprints
A shorter Sprint generates more learning cycles and confines the cost of being wrong to a smaller window, which is why each Sprint can be treated as a short project in its own right. Stretch the horizon too far and two things start to go wrong together. The Sprint Goal has longer to become invalid before anyone acts on it, and complexity accumulates faster than the team can inspect it.
Cancellation
A Sprint may be cancelled if the Sprint Goal becomes obsolete. Only the Product Owner has that authority. Cancellations are uncommon, because Sprints are short enough that finishing usually costs less than stopping.