Stow-on-the-Wold is busiest in summer (June to August), when the Cotswolds draw large numbers of visitors and the town can feel crowded. Spring and autumn are quieter and often equally pleasant — the light on the Cotswold stone is particularly good in October.
The twice-yearly Stow Horse Fair brings large numbers of travellers to the area in May and October. The fair has taken place for centuries and is a genuine social event for the Romani and Irish Traveller communities, though it has also generated controversy and increased policing in recent years. It is worth knowing about if you're planning to be in the area on those dates.
Winter is cold — the town sits at 800 feet and the local verse has it right: "Stow-on-the-Wold, Where the wind blows cold." But it's also quiet, and the antique shops and tea rooms are still open. Christmas markets and seasonal events make December a reasonable time to visit if you don't mind the chill.