Lughnasadh has an astronomical tie-in since it's a cross-quarter day, the approximate half-way point between the summer solstice (June 21) and the autumnal equinox (September 22). Here's a table showing the length of the day (in hours and minutes) and direction of sunrise/sunset with respect to a due east-west meridian. June 21, the summer solstice, is the longest day of the year and the time when the Sun rises in its most northerly position. September 22, the autumnal equinox, is the time when day and night are approximately equal length at 12 hours each (equinox literally means "equal night") and the Sun rises almost due east-west.
Where did I get this data? Because of the shape of the Earth's orbit, these numbers don't change at equal rates throughout the year. Want to learn more, take my Ancient astronomy course!
Wiccans and other neopagans today, as in the past, celebrate the solstices, equinoxes, and cross-quarter days (sabbats) which are traditionally named Yule, Imbolc ("im-bulk"), Ostara, Beltane, Midsummer, Lugnasadh ("loo-nus-uh"), Mabon ("may-bon"), and Samhain ("sow-en" with sow rhyming with cow although there are variations in pronunciation).
Neopagan Wheel of the Year
Why is Christmas in December? It's not because there's any evidence that Jesus was born on December 25. It was an attempt to supercede the ancient pagan celebrations of the winter solstice - a time the Sun is reborn and days start to grow longer again. Notice the similarity between Ostara and Easter? The name of the most holy day of the Christian calendar comes from the ancient dawn goddess. It also falls around the vernal (spring) equinox. The Medieval church changed Lugnasadh to Lammas or "loaf-mass day" where farmers brought a loaf to church from the first of their wheat crop. Samhain became All Saints' Day in the Catholic church. Imbolc was taken over by the church and called the Feast of the Presentation of Jesus at the Temple or Candlemas. (today we just "celebrate" Groundhog Day - a practice rooted in Germanic paganism).
I have to confess an affinity for the pagan calendar since it makes such intuitive sense for those of us in the mid-latitudes since it's based on the cycle of the seasons - the astronomical variations in the length of the day and position of the sun. It's a cycle that was observed by virtually all pre-Christian cultures around the world -- some of whom built megalithic (large stone) structures like Stonehenge to observe and celebrate them.