Late spring and early autumn are the best times. June is particularly attractive because the cathedral hosts its summer son-et-lumière, "Les Nuits Lumière", on warm evenings — laser projections trace coloured lines from monument to monument across the old town. The show runs from June to mid-September.
Late April brings the Printemps de Bourges, one of the longest-running rock and pop festivals in France: a week of concerts in venues all over the city. Book hotels months ahead if you want to attend.
Summers are warm but rarely uncomfortably hot; winters are cool and grey. The cathedral interior is cold year-round.