When something is occasional, it happens now and then, but not on a timetable and not as the usual state of things. An occasional visitor comes some weeks and not others. Occasional rain shows up, then goes away. It sits between “hardly ever” and “often.” In real talk people also use it to downplay: “I have the occasional cigarette” can mean they smoke more than they want you to think, or that they want the habit to sound small. The word is ordinary in speech and writing; it does not feel fancy. Though actually, it has a quieter second life: an occasional table is a small extra table you pull out when you need it — here occasional means “for this or that use,” not “rare.” Learners almost always meet the “sometimes, not regularly” meaning first.