Dundee enjoys one of the driest and sunniest climates in Scotland — it sits in the rain shadow of the Highlands and the Sidlaw Hills — so the city is a sensible destination even when the west coast is socked in. May through September is the prime window: long days, mild temperatures (mid-teens to low twenties in Celsius), and the gardens and Tay countryside at their best. June and July are the busiest with festival programming and graduation ceremonies.
Winter is cold but not brutal — typically a few degrees above freezing during the day, with occasional snow and frequent wind off the North Sea. The Christmas market in City Square is a small but pleasant fixture in early December.
Time a visit for the Dundee Design Festival in late May, the Dundee Mountain Film Festival in November, or the Discovery Film Festival for children in October-November. The big civic event of the year is the Light Night switch-on in late November.