明日休みますって英語でなんて言うの？

You never really have to go into much detail unless asked directly, so you can simply say "I'll be out," or "I won't be in tomorrow." If you feel it is necessary, you can add that you're going to be out for "personal reasons." There isn't anything wrong with saying "absent," but it is used more with talking about school. So if you were going to miss school, you could say "I'll be absent" and it sounds more natural.

It depends who you are speaking to how you wish to explain this scenario. If it is a coworker, there is no need to give any explanation unless you particularly want to. In the UK, many people regard their private lives as their own and may choose not to give any details about their absence. "I'll be off work tomorrow." However your boss may require at least a partial explanation and so you may say: "I'll be absent from work tomorrow as I have a personal matter to attend to."

Absent is a formal word that means you will not be present somewhere, in this case, work. You can just state that you won't be in work, which is a very clear, simple message to someone. You can also add on to your statement to say that it is due to personal reasons. You can be specific if you want i.e. due to a wedding, or a death in the family, or a job interview. It depends on how much information you wish to share.

If ever you need to be absent from work or school, you can always let your supervisor or professor know that your reasons are personal. You should first apologize for your absence and then explain that you have to tend to a personal matter. You can also simply say, "I'll be absent because I have a personal matter I need to handle."

Being absent from work can mean one of two things: either you are absent without leave or you are on one day's leave. If you are going to be on one day's leave, you have to sign a leave form. Being absent without leave is usually an offence that may render you going without pay for that day. In certain cases, you may be liable for disciplinary action if it is done repeatedly. However, applying for a day's leave in order to sort out an important family issue is the accepted method in most companies. It is regarded as family responsibility leave.

If you was talking to a friend you could shorten it down and just say " I won't be coming into work tomorrow" But your boss might ask for more details as to why.

Unforseen circumstances are Beyond our control...Yhey are unplanned and so they are also unavoidable.... "I will be away from work due to unforseen circumstances.."(.Beyond my control.)