Correct spelling, explanation: the base word in occasional is occasion. This is a word of Latin origin, and in its earlier form, it was spelled occasio. Of course, the word has changed slightly as it adapted to English, most notably, the n at the end was added as it’s not so natural in English that the words end in o. It has also gained the -al suffix to make it into an adjective, but besides that, not much has changed in occasional in all this time.

Definition of occasional:
adjective
1. happening rarely, only in particular situations
In general, the weather here is sunny, with some occasional rainy days in the autumn.
She doesn’t drink alcohol, except for occasional craft beer when she’s on outings with friends.

Incorrect spelling, explanation: occassional is incorrect, but it is definitely easy to get lost in how many c’s and how many s’s should be there. The correct spelling is occasional, with just one s, just like it was spelled in Latin – occasio. But, more realistically, when you struggle with occasional, think of the base word occasion – you probably wouldn’t put a double s there, so keep it up when you add suffixes!

Incorrect spelling

Incorrect spelling

Incorrect spelling