Susan - you're right. The missing one is Imbolc, celebrated at the start of February. That's followed by Beltaine (aka Bal's Fire, and more latterly, May Day), then Lughnassa (sp?) and Samhain (aka Halloween (All Hallows Eve) or All Saints Eve. Most pre- or non-industrial societies had/have dates of similar significance on their cultural/spiritual/religious calendars.

Samhain was the Celtic New Year's celebration, when the fires of the old year were extinguished and the new one lit. Folk's had to stay up all night lest the evil spirits snatch them away, and usually did so in the context of heavy-duty feasting and partying - the forerunner of the western world's current approach to New Year's Eve (the current habit being a much enfeebled version of the original).

If a person is so inclined, you could likely have a "legitimate " New Year's "event" at least four times a year, and maybe more.