Eilat is one of Israel's sunniest and warmest cities year-round. Winters (December to February) are mild and sunny with temperatures around 20°C — this is when most Israelis descend on the city to escape the cold north. It is the busiest and most expensive time to visit.
Spring and autumn are the best seasons for outdoor activities: comfortable temperatures, clear water, and somewhat lower prices. Spring (March to May) also brings the remarkable bird migration — millions of birds pass through Eilat on their way between Africa and Eurasia, making it one of the world's premier birdwatching destinations for a few weeks in March and April.
Summer (June to September) is brutal: daytime temperatures regularly exceed 40°C, the beaches are packed with Israeli families on school holidays, and prices peak. Unless you plan to spend most of your time underwater, summer is the season to avoid.