Hildesheim is worth visiting in any season, but May is the most atmospheric time: the thousand-year-old rose in the cathedral cloister is in bloom, and the days are long and mild. The Christmas market in the market square (December) is popular with day-trippers from Hannover and is one of the better markets in the region.
Summer is pleasant for outdoor sitting in the square and makes for comfortable sightseeing in the churches. The churches themselves are cool inside and a relief in hot weather. Weekday mornings are the quietest time for the cathedral and St. Michael's, before the coach groups arrive.