May to early September is when Rakvere is at its best — the castle is fully open, the cafés put tables on Pikk Street, and the long northern daylight stretches into the evening. Mid-June around the Pühajõe music festival is busy; the Green Fest in early summer and the Punk Song Festival (yes, really, every two years in the castle moat) are the kind of small-town events worth timing a trip around.
October to April is cold, dark and quiet. The castle's outdoor activities close down; some restaurants keep limited hours. A Christmas-market weekend in December is pleasant if you're already in the Baltics.