Beppu is a year-round destination — the hot springs operate regardless of weather, and the onsen experience is particularly appealing in winter when the steam is more dramatic against cold air. Spring (late March to early May) brings cherry blossom to the hills around the city. Autumn (October and November) offers excellent foliage, especially on Mount Tsurumi accessible via the ropeway.
Summer is hot and humid; the beach sand baths are particularly popular July through August when domestic tourism peaks and prices and crowds are at their highest. If you prefer a quieter visit, November through March outside of the New Year holiday is the best window, with cooler weather making the hot springs more enjoyable.