Both Wiccans and Pagans tend to celebrate what we call 'sabbats'. (Maize's link is excellent for advice.) As Master Vigil mentioned, many also celebrate or honor the phases of the moon. In addition, if the practitioner feels a connection to a certain god or goddess, they may also celebrate days specific to that diety. For example, I love Artemis, so I 'party hearty' on February 7th and 8th. Lots of other dates can be found here (http://www.wheeloftheyear.com/2003/pagan.htm).
*nods* Although I am not Wiccan, many Pagans do this. Other holidays that are celebrated are from the Greek, Hindu, Shinto and First Nation traditions/calendar.
It is considered that the celtic year was eight fold, you mentioned Samhain (which is my birthday!!!!) but the year runs from the celtic new year Samhain, followed by the winter solstice, then Imbolc, then spring Equinox, then Beltane, then summer solstice, then Lammas, then autumn equinox, where upon arrival again at samhain the wheel finishes turning for a year. Apart from the two solstices and two equinoxes the others are known as fire festivals.
Here is an interesting concept...reversing the holidays for the southern hemisphere. It is obvious that the sabbats should be reversed on the calender (so our yule is actually in winter), but what about the holidays? it seems a bit silly to be celebtating Beltaine in autumn approaching winter, but many believe that they should not be altered. Me personally, id as soon spend Beltaine outside in the cold as jump off a cliff!