Hey there Kimmy

As a name, it is Welsh in origin, and literally means 'son'

In Earth-centred belief sysytems (which include amongst many pagan and wiccan), Mabon may be interpreted as 'son of the Great Mother'.

Some Earth-centred beliefs focus on only the Earth as a spiritual entity, some have the Goddess and others have the Goddess and her male consort in a duality of partnership.

Mabon is not celebrated or observed by all... but those that do see it as a time when the male part of the spiritual equation retreats, and prepares for the "rebirth" of the Goddess in spring.

As far as "holy days" go in the pagan calendar, this is a more recent addition, rather than a long standing observance like the other 7 days in the year.