The word occasional means that something happens or shows up infrequently. It implies that there is no schedule.
Ex: I attend an occasional art show. I enjoy seeing what new artists are creating, but I would much rather spend the time reading.
Ex: Because I only gave occasional attention to my Chemistry notes, I am in danger of failing. That is detrimental to my grade point average.
The word occasional can also refer to something that is only used at certain times. It can also be used to describe something that is developed around or for a certain occasion.
Ex: I have an occasional table that I will put by the front door. It is painted red and green to make the most of the Christmas spirit.
Ex: My music teacher handed out the words to the occasional songs we needed to learn. She always seizes the opportunity to sing holiday songs.
The word unplanned refers to an activity or state that has not been thought through. It implies that the person did not intend to participate. In this case, it is a synonym for casual.
Ex: I believe it was an unplanned pregnancy. Although they are thrilled, the Smiths already have three kids and had not intended to have more.
Ex: I eat the occasional bowl of ice cream. It is usually an unplanned purchase that I make if I am really in the mood for something sweet and cold.
The prefix un-, meaning "not", can also be used to make other common words. For example, it is used to form the words unfair and unforgettable. The word unfair means "not fair" or "unjust". Unforgettable means "not able to forget".
Ex: Everyone thinks that our math professor is unfair. He seems to delight in changing his grading method at the last minute.
Ex: That film was unforgettable! The lead actor was a shining star, and the message will certainly stick in the mind of anyone who sees it.