Beltane, co-opted by our mainstream culture as May Day, is a holiday for love, fertility, and growth. If you look at the cycles of nature that our agrarian pagan ancestors would have constructed the rituals around it makes perfect sense. The earth itself seems to be procreating in every way, and so do animals. One of the best known celtic rituals of Beltane involved the driving of livestock to their summer pastures. In this process the animals were driven between two ritual bonfires so the smoke could purify them of diseases and any bad spirits that had attached themselves to the creatures over a long dark winter. A feast day and ritual activities accompanied this purification rite and most of it had to do with fertility. It was and is a traditional pagan day for handfasting, or marriage vows to take place. The phallic maypole driven into the feminized earth makes the intent of rituals pretty clear. Beltane is a time to honor the beauty and hope of life going on. It is also a time for some ritualized lustyness and general messing around. Demurely referred to as "Maying," this is the aspect of the holiday that echoes in modern folk practices of hanging may baskets on the door of sweethearts.
Vegan Pagans can honor Beltane by celebrating the impulse of all things of this earth to procreate and continue...to live. Along side the commonly known Beltane traditions, you can veganize the holiday by harnessing the new birth/growth energy to take a veg pledge and tune up your animal consumption patterns if there are still things you want to change. You probably know this but http://www.peta.org/ has a great veg challenge always available.
Sending donations to an animal sanctuary that cares for rescued creatures and their young is another great new tradition. Look for a local sanctuary or visit ones like http://www.farmsanctuary.org/