When visiting Rivas it is important to take into account the rainy season, which typically runs from May to October. The city of Rivas gets less rain during rainy season than surrounding beach towns and temperatures are more tolerable in the city.
Temperatures are hot from November to January, but not unbearable. Because of the low altitude and distance from the ocean, the heat peaks in Rivas during March or April, and visiting is not recommended.
If you do decide to endure the heat, try to pass through Rivas during the Semana de Dolores, which is the week before Semana Santa (Easter Week). Go see the annual event, Peregrinación de las Carreteras, when people come in carriages come from all over Nicaragua to Rivas to celebrate Jesús del Rescate. (This is an image of Jesus in the Iglesia de San Francisco.) The festivities usually last all week. During the religious festivities, reserve a place to stay in advance.