May through October is when Cumberland makes sense as a destination — the steam railroad runs, the towpath and GAP are open and dry, the state parks are in full swing.
October is the standout month. The fall colours in the Allegheny Mountains around the Narrows are exceptional, the cycling weather is perfect, and the steam railroad runs special leaf-peeping excursions on weekends. Hotels fill up — book ahead.
June through August is reliably warm and humid. The Lake Habeeb beach at Rocky Gap is busy on weekends, and the GAP/C&O cyclist traffic is at its peak.
Winter is quiet and cold. Snow on the mountains is possible from November through March. The railroad runs a small holiday series in December but most other things close down.