Ardee is worth visiting any time of year, but late spring and early summer — May through July — bring the best combination of long days and reasonable weather for exploring outdoor sites like the Jumping Church at Kildemock. The castle museum keeps standard opening hours and can be visited year-round.
The Táin Festival, held in County Louth in summer, celebrates the Ulster Cycle mythology and occasionally brings events to Ardee given its connection to the Ferdia legend. Check local listings if the mythological angle interests you.