South Limburg has a slightly milder climate than the rest of the Netherlands — the hills moderate the wind and summer temperatures are a degree or two warmer than in the flat north. Spring and summer (April–September) are the best months for walking and cycling the surrounding countryside.
There is no strong seasonal reason to visit Heerlen specifically. The indoor attractions — Thermenmuseum, Glaspaleis, Schunck — are year-round. The Pinkpop Festival, one of the oldest pop music festivals in the world, takes place annually in the neighbouring town of Landgraaf in June; it draws large crowds and accommodation fills up in the wider region.
Winter is quiet but the Christmas market in Maastricht, a short train ride away, is one of the best in the Netherlands and worth combining with a Heerlen visit.