Spring (April–May) is the sweet spot — pleasant temperatures, full water in the Ouachita rivers and waterfalls, the Garvan Woodland azaleas in full bloom, and the climax of the Oaklawn racing season. The Arkansas Derby in mid-April is the busiest weekend of the year; book hotels well in advance.
Summer is hot and humid, with daytime highs in the low to mid 30s C. The lake-based half of the visit (Hamilton, Catherine) makes sense in this season; soaking in 40°C bath water less so. Autumn brings cooler air and decent foliage colour on the surrounding mountains, especially in late October. Winter is mild compared to the rest of the country — daytime temperatures often hit 10–15°C in January, ideal weather for a hot soak — but bathhouses and outdoor attractions run shorter hours.