'Hogmanay' is the Scots word for the last day of the year - roughly equivalent to 'New Year's Eve'.
It signals end-of-the-year / beginning-of-new-year celebrations - which traditionally stretch into New Years Day.
The origin of the word in unknown. It dates back at least as far as the mid 1400s. There are several differing theories - that it may have originated from Greek, French, Goedelic (Gaelic), or Norse languages.
See : Wikipedia