The set of all months that contains less than 31 days is; {February, April, June, September, November}.
It follows from the task content that the set whose elements are to be determined is characterized by containing less than 31 days.
On this note, it follows that the elements of the set are; February (28/29 days), April (30 days), September (30 days), June (30 days) and November (30 days).
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