Pagan paths are very diverse, but most British Pagans tend to celebrate eight or nine festivals through the year - which we liken to a wheel. There are two main groups: the four solar festivals and the four seasonal festivals. The former are the solstices and equinoxes, fixed by the Earth's orbit around the Sun. The latter originated in agricultural society and are traditionally celebrated at key seasonal changes. It was the Christian Church which fixed dates for these when it super-imposed its own festivals on top of them. We often use the old Celtic names for the seasonal feasts, although often inter-changeably with the Christian names which are of course common parlance. Many Pagans celebrate on the Church-ordained date or on a close weekend for convenience, especially when living an urban lifestyle and in situations where being openly religious (let alone Pagan) is still difficult or even impossible.
The fixed dates for the seasonal festivals stay the same each year: Imbolc (2nd February), Beltane/May Day (1st May), Lammas/Lughnasadh (1st August) and Samhain/Hallowe'en (31st October). But these festivals are seasonal, so many of us actually celebrate when the season changes, as marked traditionally by the first snowdrops (Imbolc), the hawthorn (may) blossom (Beltane), the beginning of the grain harvest (Lammas) and the first real frost (Samhain), which can vary around the country and from year to year. (Naturally, these festivals apply to the temperate North, and really only to Britain.)
The four solar festivals (the Spring Equinox, Summer Solstice, Autumn Equinox and the Winter Solstice) fall at the same time each year (as one would expect), but the exact date can change a little as our calendar is slightly out of sync with the Earth's orbit of the Sun. They do however generally appear in conventional diaries as the 'first days' of the four seasons. Thus, in 1999, the Winter Solstice fell on the 22nd December, whilst in 2000, it falls on the 21st. The Winter Solstice is also the start of Yuletide, which is celebrated in various ways:
Note also that many festivals do or can start in the evening. Samhain is the most obvious of these, which is why the date assumed is 31st October rather than 1st November, but also why Imbolc can be seen noted as 1st February and Beltane as 30th April. Midwinter and Midsummer Eves can be the nights before the Solstices, or more traditionally, the nights before Midsummer's and Midwinter's Days.
These are the main festivals making up the modern Pagan 'Wheel of the Year' here in Britain. There are others which are celebrated by particular traditions and paths, by those with dedications to particular Gods and/or Goddesses, and many personal and local community festivals. There is also Earth Day (or sometimes Earth Healing Day) in the second half of April, which is a recent introduction from the U.S.A., an international, secular day, but which seems mainly to be marked in this country by Pagans.
In summary, the commonly celebrated Pagan festivals are (solar dates correct for 2000/1):
For more information on Pagan and other traditional festivals, read the following books: