I think most freelancers originally worked as an employee at some time in their lives. As an employee, it becomes a habit (by necessity) to tell your employer that you are planning to take a vacation, or you need sick time, or family leave.
Once you become a freelancer (contractor), please keep this in mind: your client is not your “employer”. Any arrangement between the two of you requires your consent, not just theirs. It is “just my opinion”, but you should not feel a need to justify your schedule to a client, unless you’ve signed a contract which says you will do some specified piece of work for some specified hours during some specified time period.
Here is one example: A client hires you to complete a project in July of this year. Once the work is done, your relationship with them is officially finished. Don’t feel you need to tell them “I have a vacation scheduled in September”, just in case they might need you again at that time. That is none of their business! Once you start doing things like that, it plants a seed in the client’s mind that you will be available unless you warn them otherwise!
However, if you detect some expectation that they might be planning or assuming that you will be available for future work, you may want to caution them that you will not be available going forward without an agreement in place. For example, suppose you finish your batch of work, and the client emails you something like “We may need help in creating more content in the fall of this year.” You can send an email back saying “I am not sure of my availability at this time. Please send me an email with further details when you are more sure of the schedule and the amount of work needed. If I am available, I would be happy to help you.” You might want to mention a retainer agreement at this time.
Suppose the client thinks that they might need you for additional work in September. In that case, they should make an agreement with you in advance that you will be there for them – this is a “retainer”. So, they pay you $$ in advance so that you carve out time which is reserved for them – and the agreement specifies that you keep the money whether or not they use your services. Remember, they are reserving your time with the payment. You could have been working for someone else during that time period, if you had not reserved it for them. You should not be losing money because you set aside time to work for them but they were too disorganized to make use of it.
So, if you finished your work in July, and the client signed off on it, they have no right to send you an email at the end of August which says “We need you for work in September.” It’s actually kind of rude to do this. They’d be making an assumption that you are twiddling your thumbs, waiting for new work from them, which is generally not the case for freelancers.
Two of the things you need to do as a freelancer is to “manage client expectations“, and “set boundaries“. Boundary-setting means the client doesn’t get to know if you are on vacation, giving a work seminar in a remote location, or anything else. It is not their business. (I am speaking from a USA-centric point of view, fwiw).
Managing client expectations, in this respect, just means that the client sometimes needs to be reminded that you are not an employee. Managers who are only used to working with employees sometimes forget that contractors are not machines that are instantly available. Sometimes, they are used to pressuring contractors to behave like employees, with the implication that they will not give them future work unless they are always on call, respond to emails instantly, etc.
It is up to you how you respond to such pressures, of course. In the long term, my belief is that you are better served by pushing back when clients start to act like your “boss” – because they are not! There’s a risk that this client might “fire” you, and get someone else to bully (good riddance!). Hopefully, though, they will learn to be respectful of your time.