May and June are the classic months: forests in fresh leaf, lakes warming, and — on the Tuesday after Whitsun — the Dancing Procession (Springprozession), a UNESCO-listed pilgrimage that fills the town with tens of thousands of visitors. Book accommodation months ahead if you want to be there for it.
July and August are warm and busy, with the Echternach International Music Festival running through the summer in the basilica.
September and October are quieter and the Mullerthal forest is at its best — autumn colour through beech and oak, dry trails, and reasonable temperatures.
Winter is closed-feeling: many small businesses close, the Mullerthal trails can be icy, but the basilica is open and the town has a quiet appeal.