The links above connect you to a great set of seasonal pages from Waverly Fitzgerald's School of the Seasons. You may also wish to visit the main page for this set, which has lots more seasonal information, including other holiday ideas and lore. These pages are full of seasonal spirit, and include the Celtic cross-quarter (mid-season) festivals as well as the Germanic quarter (start-of-season) ones. Starting from top center and proceeding clockwise, they are: Halloween (Samhain), Yule, Imbolc (a.k.a. Candlemas and observed either Jan. 31 - Feb. 1 or Feb. 1-2, depending on your source -- either way, Celtic days begin at sunset), Ostara (Spring Equinox), Beltane (May Day), Litha (also called Midsummer, though it comes at the start of the season), Lammas (Lughnassa), and Mabon (Fall Equinox). And so back to Samhain (which is pronounced either SOW-en or SAHV-en, by the way, depending on whether you follow Irish or Scottish pronunciation).
For all the interest in the great myths of gods and goddesses, it appears that much of Celtic mythology focused on the various genii loci. Certainly, Roman commentators observed that Celtic Britain was littered with shrines to these resident spirits, along streamsides, at crossroads, etc. Following this tradition, you may wish to visit my own virtual shrine to one such genius loci, here in my own neck of the woods.