We are going to Slovenia next May, arriving right at the end of April. I have read online that Slovenes really celebrate May Day: with bonfires the night before, brass bands in the morning, and erecting May Day poles that boys try to climb. Sounds like fun. My question: are we likely to see more of this in smaller towns and villages? We had been planning to start the trip in Ljubljana.
Yes, May Day is celebrated throughout the country although the holiday itself is traditionally spent having a picnic with the family or going on a family day trip to a nearby place of interest. Bonfires are held the night before, with a large one on Roznik Hill above Ljubljana, too. Maypoles are erected all over the country and brass bands play in the morning to wake inhabitants up - depending on the level of partying the night before, it can be either lovely or quite annoying. :) In some areas with ironworking traditions, for example in Bohinj, it's also customary to fire mortars in the morning.
If you wish to experience the whole thing, it's best to be located in a smaller town rather than in Ljubljana, although Ljubljana will have its fair share of activities. I'm not sure which town has the most comprehensive and tourist-friendly set of festivities, though. If you don't get a useful response on the other forums you've posted (TripAdvisor has a lot of locals on their Slovenia forum), I suggest contacting the tourist office to see if they have more information on this matter. You might not get any useful information at this point, though, as it's still quite early. Communities won't start publishing detailed information on next year's festivities until a few weeks before May Day, but you'll find some festivities nearby wherever you're staying.