A day of such supremely good weather that it may be necessary to skip work or class.
As opposed to INclement weather days, when schools and businesses close due to snow or other harsh conidtions.
"Today has been declared a clement weather day, and employees will be dismissed early. With today's 75 degree temperatures and cloudless skies, all caution should be taken to avoid the indoors, as it would most likely be detrimental to your health. This has been your local weather watch."