Panama has two seasons: dry (December to April) and wet (May to November). The dry season, locally called verano (summer), is the more comfortable time to visit — lower humidity, clear skies, and cooler mornings. January through March is the peak of it, and the best time for walking Ancon Hill and cycling the Amador Causeway.
The wet season is hot and humid with afternoon downpours, but it rarely rains all day. Mornings are often clear. The Canal operates year-round regardless of weather, and the locks are equally worth seeing in a light rain. If anything, the wet season landscape is more lush, which makes Ancon Hill greener and the wildlife more active.
Ship traffic through the Canal is consistent throughout the year, so there is no significantly better or worse time to visit Miraflores for that purpose. December to April sees more cruise ships on the Pacific side, so weekends during that period will be busier at the locks visitor centre.