According to Frommer's, there is no true rainy season in Aruba like there is at more lush vacation spots, such as Costa Rica. Aruba averages only 18 inches of rainfall a year, and while most of it does fall between October and January, showers are often brief and sporadic.
According to Frommer's, the high and low seasons for tourism correspond to the weather in North America more than they do to the weather on the island itself. When it is winter in North America, demand is high for warmer environments and sunshine peaks, bringing more people to Aruba. The busiest months are typically mid-December to mid-April.