There are some biggest music festivals in Hungary happen during summer every year and millions visit the festival. These are
Sziget Festival (August)
Balaton Sound (July)
Volt Festival (Jun/Jul)
Veszprem Festival (july)
Hungary also hosts formula 1 race in Hungaroring (mogyrod) during end of July .
Christmas Markets (Dec)
You can find several christmas markets during December in Budapest city centre serving hot mulled wine.
Wine and Beer Festival (Jun/Jul)
Wine and beer tasting festival happens in Castle district during June/July every year.
Please see this page for complete list of festivals in Hungary: https://welovebudapest.com/en/toplists/2017-calendar-of-festivals-and-events-in-hungarys-countryside/