Happy Beltane to you too!

I don't know the precise answer to your question, but I wanted to throw some food for thought out myself. Mostly, I am familiar with May Day being the first of May and the 'witches sabbat' being the night before from my German roots (Walpurgisnacht).

But I also check out guides like this one: www.archaeoastronomy.com/2008.shtml

to also see exactly when the astrological energies are at their peak for an Equinox, Solstice, or cross-quarter day.

Also, Beltane, like Samhain, being a big deal around an 'elightened' place like Asheville, the festivies tend to stretch out over a 2 or so week period, encompassing both traditional days like April 30th, and astrological days like May 4-5th. This works out great for community gatherings (which also take into consideration practicalities). But if individuals feel like April 30th is better then May 4th, or visa versa, then they can always do what they need to do on their own.

I try to resist the temptation of being too dogmatic about the days and times for rituals. May day (itself) is probably no older than a couple hundred years as a folk tradition, but we are fairly certain that groups of people gathered together from neighboring towns to celebrate the spring for eons before that. It seems to have been a chance to meet a new lover for a summer romance. They may have celebrated on the eve before Beltane or many evenings later, as was convenient for the people and as the weather cooperated. To me, it is important to ackowledge the event by celebrating it, and when we celebrate it is secondary.

The Universe doesn't seem to be in sync with our calendar used here in the USA. It needs us to put an extra day in every four years to make it work. That tells me to ignore it unless I am dealing with the mundane daily life I must encounter to earn my rent and food. When it comes to celebrating and other sacred matters, timing is between me and Spirit, the spirits of place, and the timing of the Universe itself for spring, or any other natural event. Nature is not rigid, trees flex in the wind, water courses around solid objects. The patterns of life are like those cast by the shadows of the sun as it plays with the leaves on a breezy day. For sure the shadow is cast, but it dances with the leaves as the sun plays upon the leaves.

Beltane is not on May 1st, that is the Christian calendar . . . Beltane is a Cross Quarter holiday and has nothing to do with our current calendar system . . . basically the quarter holidays ( all fall exactly half way between the solstice and the equinox) . . . this is also the same in many indigenous cultures and corresponds with early Native American astrological time-keeping from rock art and pictographs . . . In Europe this would coincide with Neolithic and early Bronze Age when megalithic monuments were sued for record keeping, such as Stonehenge, New Grange and Knowth Howe

www.glenboycountryaccommodation.com/loughcrew.htm
Cairn U, north east of Cairn T, has a stone bisected by a shadow at Summer Solstice, and its passage is aligned to 4 February and 8 November cross-quarter days, like the passage tomb at Tara. Cairn V, south east of Cairn T, marks the winter solstice. Y-shaped Cairn S, north west of Cairn T, marks sunsets on 6 May and 8 August cross-quarter days. A stone at Cairn N, south west of Cairn T, aligns on the Hill of Fore to mark the winter solstice sunset.

en.wikipedia.org/wiki/Cross-quarter_day
homepage.ntlworld.com/mjpowel...ites.htm

You can celebrate the Cross Quarter holidays whenever you want, but if you are trying to use the energy of the season the actual dates will depend on the sun's position, not a calendar (kinda like working with full moon on a more convenient night). . . personally, I do ritual for a full week surrounding these eight earth/solar holidays, working with the energy for several days before and after.
AP

"Now for the question, how do we know that beltane is dusk april 30 to dusk may 1?" Sorry, I didn't fully answer your question . . . the ancient Celts reckoned time from sunset the day before, so sunset to sunset , just as they counted the new year from winter to winter . . . the dark before the dawn . . . seeds sewn in the dark will be harvested in the the light . . . AP