Frankfurt has a very temperate climate. Temperatures are not extreme and not unlike Berlin, the most rainfall occurs during the summer, as you can see on the historic climate charts below, but summer offers only moderate rainfall. Winters can be bleak and stormy.
Frankfurt Airport is a major airline hub for Lufthansa.
June through September are Frankfurt's most popular months for travel. But even if you don't intend to stay there, Frankfurt's airport is a major international transportation hub, so it's likely you'll come through if you're visiting other destinations in Germany.
Spring is a popular time to go to Frankfurt, particularly March through May. You might be subject to quick and occasionally heavy rain showers, but for the most part you can expect good weather. As besides, there are many fine museums you can duck into when it rains.
The charts below show the historic high and low temperatures in Frankfurt and the month by month historic rainfall.