Things happen that you cannot predict. A global pandemic, earthquake, computer hard drive crashing, virus, travel, new game coming out.
Some of these you can predict, prepare for, and manage in advance. Others you cannot. Allow for contingency time where nothing specific is scheduled.
At worst this time can be used to refined and further test your final outcome.
Schedule contingency time throughout your project.
Investigate critical paths for Gantt charts to determine which tasks must be completed at certain times.