It is important to choose the right time to make a functional change on an existing Microsoft Dynamics NAV or in Microsoft Dynamics 365 Business Central implementation. Some functional changes can be deployed at any time, others not. Some of them may require a lot of time and no users to be working and changing data; you may want to choose a weekend for that. Some others could be implemented at any time, but to keep a better track of the time at which it began to work differently, you might want to choose the start of a fiscal year or the start of a month.
The important thing is to think about it, analyze it, and choose the right time for every functional change implementation, as a wrong choice could cost you dearly. It is always useful, in cases with complex procedures, to draw up a risk analysis in order to have a way out in case of problems that could arise.