You should of course follow @preet-sangha's answer and send a nice mail explaining the situation. Just know that he/she is under no legal obligation to accept this, and could even fire you for not showing up to your job. This is not reasonable or likely; it borders on the insane to refuse this to an employee. However, the law does give him/her that power.
Therefore, just for completeness:
In Belgium, there are some situations where your employer cannot deny you vacation time. When a family member dies, or when you get married. The complete list is below, and it is highly likely that other countries have similar laws (link in Dutch):
Death of a spouse or child: 3 days
Death of a close family member: 2 days
Death of a family member: 1 day (the funeral)
Marriage (your own): 2 days
Marriage (of a close family member): 1 day
Holy confirmation (growing-up ritual of your child): 1 day
Civic duty: as much as is required
Inauguration as priest of a close family member: 1 day
Recruitment into the army, or as a conscientious objector: as much time as required
Going to the peace court: 1 day
Civic duty as a juror: as much time as required
Taking care of a sick family member: min. 1 month, max. 3 months
Anything else can be refused by your employer, but would certainly stress the professional relationship you have.