Is Imbolc one of the cross quarters?
Yes. Thus it is a Celtic fire festival and a Wiccish festival. (Wiccan/Wiccan-derived other.)
Also, I know that certainly not everyone uses the wheel of the year, but many people seem to celebrate holidays at the same times. So what calendar do you use? I mean, do you consider Samhain to be the new year, etc.
Samhain is also a cross-quarter, and thus a Celtic fire festival and Wiccish.
My liturgical new year falls in the summer; it originally coincided roughly with the rising of the Nile, and dates from the heliacal rising of Sirius. (Precession has unlinked these now.) Samhain is from the Department Of Other People's Holidays, like Imbolc.