Kaiser Imperial Ball, Hofburg Palace in Vienna, held from 31 December to mid March.
The best music festivals, whether they be jazz, folk, opera or rock festivals will be high octane, pricey and crowded. Best to buy tickets as soon as they go on sale though some ticket sellers pass on most of their stock almost immediately to affiliated resellers who then sell them for a higher price.
Furthermore, some touts use high speed, multi-address software to grab all the tickets online as soon as they become available. Hmmm. p. s. take plenty of water with you!
Calendar Dates are sometimes unconfirmed and occasionally unreliable, so check before booking. TBC/A = To Be Confirmed/Announced