Derry is a year-round city. The most notable event in the calendar is the Hallowe'en festival in late October — one of the largest free outdoor festivals in Ireland, with fireworks, parades, and costumed events that have built an international reputation. Book accommodation well in advance if visiting then.
The St Patrick's Day celebrations in March are also large by Northern Irish standards. Summer brings more daylight and better weather for walking the walls and the Bogside, but the city is never very crowded. Weather is mild but wet; bring rain gear regardless of the season.