Pagans of yore typically held a Lammas (also known as Lughnasadh) celebration the first week of August to mark the beginning of the harvest season. Some say that the tradition dates back to early Ireland, when farmers wouldn't dare harvest their grain before Lammas.
That would mean bad luck, as the summer's bounty would have run dry early, and bode poorly for the food stores. If the farmer cuts his first sheaves of wheat on this day, his wife then bakes the first loaf of bread, which gives rise to the name of the ritual, "half-maesse," or loaf mass, an ode to the bread.
Pagans today usually look at this festival as a celebration of abundance, an Indian-summer feast of farewell to summer and a welcoming in of the cooler days of autumn. It is also a time of honoring the cycle of life, death, and rebirth.
Because of this time of transition, foods eaten at a typical feast can include late summer produce as well as early fall bounty such as green corn and grains, along with freshly-baked bread. Rich soups and hearty dishes including summer squash are often shared as potluck.
The colors would include late summer, early autumn colors of green, yellow, and rust. Since Lugh was a craftsman, you can also include crafty items on your altar.
Some pagan groups will have a circle ritual, in which the spirits of the four directions (earth, wind, fire, water) are called in, and a circle is created with the physical bodies of the participants; and some reading, poetry, etc., that designates the purpose (celebrate abundance, harvest, honor the "corn mother," or "Earth Mother," etc) are performed.
Often stalks of wheat are passed around for the participants to hold, or a loaf of bread is passed; and each person takes a piece and might say something before passing it to the next person.
Luckily, with today's modern pagan groups incorporating Celtic traditions as well as Native American rituals, the group can decide exactly what kind of ceremony they would like to have.
Usually, the purpose is simply to gather together with friends and family during the time between the summer solstice and the fall equinox in September, and enjoy the changing of seasons together.