Absence explanation letter
An absence letter deals with whole days gone and no leave approved beforehand. Written as a letter rather than a form, because the thing being explained is usually why nobody could be told.
Questions people ask about this letter
Is a medical certificate always required?
That depends on your policy and on local rules, which differ by country and by length of absence. Check yours before promising one or refusing one.
Can unapproved days be converted to annual leave afterwards?
Often, if a balance remains and the manager agrees. Ask directly in the letter rather than assuming it will happen quietly.
What if the reason is genuinely private?
Say that the reason is personal and that you are willing to discuss it in confidence. That is a real answer, and most policies accept it.