The ideal time for you to visit Frankfurt is the late spring to early autumn period in the months of April and May. The summer months are usually warm around 25 °C to 35 °C with high levels of humidity and light rainfall is expected. However, winters in Frankfurt can get pretty harsh in the months of November to February with temperatures dropping as low as -10 °C with occasional heavy rainfall. Therefore, as winter fades to spring during the months of April to May, you can enjoy the tourist attractions in Frankfurt to their fullest with pleasant temperatures of 15 to 25 °C .
Keeping in mind with the year round fairs and festivals, you might also wish to avoid the busiest months in order to enjoy a relaxing vacation and still be able to find affordable accommodations.