Can I buy a Paris Visite pass (zone 1-5, days 1-3) from Orly airport on 8th May 2014? It is a public holiday in France.
The ticket office at major stations such as the airport will be open even on a holiday. In addition, there are ticket machines that accept credit cards (all of them, since this is an RATP station — on the other hand, SNCF stations such as in Roissy airport have machines that only accept credit cards with chips), and I think the ones in the airport also take banknotes (most of the ones in the city don't).
You can even buy tickets in advance from online resellers. I have no idea about extra costs and delivery times. But I'd just buy the ticket(s) at the airport.
Do check whether Paris Visite is really worth it. For many people, it isn't a good deal, but you need to match the price with what you're doing, and having a pass has the advantage that you don't need to think about cost before hopping onto a train or bus. Paris Visite zones 1–5 (covering the city, both airports and the Versailles Palace) is 48.65€. An Orlybus ticket (I recommend the bus to Denfert-Rochereau over Orlyval, it's simpler and slightly cheaper) is 7.50€ one way. A ticket to Versailles is 3.45€ one way. A book of 10 tickets for travel in the city is 13.70€. 10 trips for 3 days without counting the Versailles and airport runs is a reasonable estimate, which makes Paris Visite worth buying only if you get 13.05€ worth of reductions from Paris Visite, which is doubtful.