The Scottish Borders have a relatively dry climate by Scottish standards, but that's a low bar. Spring and early summer — May and June — offer the best combination of reasonable weather and long daylight hours, when the Tweed valley is green and the hillsides around Kelso are genuinely pleasant for walking.
July brings Kelso Civic Week, the town's main annual festival, centred on the Kelsae Laddie — a young townsman who leads a cavalcade of riders around the traditional Border marches. It is a proper local event rather than a tourist spectacle, and the week has a festive energy. The September fair draws around ten thousand people for a day of music and dancing in the square.
Race meetings at Kelso Racecourse run from September through May; March has the best card. Avoid January and February unless you enjoy raw east-coast wind.