Happy Monday; Monday national holiday
Refers to a Japanese national holiday that has been moved to a Monday under the Happy Monday System, creating a three-day weekend. The most well-known example is Coming-of-Age Day (成人の日).
wasei-eigo (Japanese-coined English), from happy Monday
See also: 成人の日
Under the Happy Monday system, Coming-of-Age Day was moved to the second Monday of January.