Archaic poetic name for the sixth month of the lunar calendar. Compare 水無月 (みなづき), the more common classical name for the same month.
See also: 水無月
常夏月 is an alternate name for the sixth month of the lunar calendar, used in waka poetry and similar contexts.
From 常夏 (とこなつ, 'everlasting summer') + 月 (づき, 'month'), referring to the hot summer month.