The phrase "occasional series" refers to a collection of related events or articles that are not regularly scheduled, but happen from time to time. Full definition
(Although Prospect New Orleans was intended to launch a biennial, cost overruns and the Great Recession have turned it into a more occasional series of exhibitions. (reliablenarratives.com)
Our friends over at The Alliance of Independent Authors (ALLi) invited me to their first Head - To - Head (a new occasional series) to debate the issues with Orna Ross, who «can not imagine any circumstances» under which she'd pay for a review. (indiereader.com)
«The Future of the Image» is an occasional series written by Jessi Hempel exploring how technology facilitates visual communication. (fortune.com)