October is Salem's signature season — the entire month is "Haunted Happenings," with costumed crowds, ghost tours, outdoor events, street performers, and a general atmosphere of theatrical spookiness that builds toward Halloween. It's genuinely fun if you embrace it, and the city makes no apologies for its commercial enthusiasm. The downside is serious: every restaurant and hotel fills weeks in advance, prices spike, and the crowds on Essex Street on October weekends can be oppressive. If you want to see the decorations and atmosphere without the worst of it, try a weekday in early October.
Summer (June through August) is the most pleasant time to visit — the harbor is active, the museum is at its best, and the city operates comfortably. Spring is quiet and beautiful; the Salem Common looks good in April and May. Winter outside of Christmas has its charms — fewer crowds, cheaper accommodation, and the historic streetscapes look excellent in the early dark. The museums are open year-round.
Avoid the Halloween weekend itself unless you have booked months ahead and have a very high tolerance for crowds.