Use 'irregularly' to describe events that don't follow a predictable pattern or schedule. The webinars are held irregularly, so check the schedule in advance.
Pro Tip 2/3
Physical Irregularities
Describes objects with uneven shapes or surfaces, not following a pattern. The cave walls were irregularly textured with dips and hollows.
Pro Tip 3/3
Unexpected Frequency
When something does not occur as frequently as expected, describe it as happening irregularly. Her blog posts come out irregularly, sometimes months apart.