Originally Posted by
Pauillac
If you are not "Skyteam elite plus" member then they will charge you the cost of a biz ticket so a minimum of 3000€

To translate that answer into more useful information: Unlike many US airlines, European carriers generally do not sell club access--it has to be earned, by flying in biz or first class or by being an elite member of their FF club. So no day passes for sale.
SkyTeam Elite Plus members can access lounges when flying an international coach itinerary. If you're a highest-tier elite on any SkyTeam carrier, that's you.