In Giessen prevails a humid-temperate climate meaning mild differences between highs and lows. There are few rainy days, fewest in March, in comparison to cities in the area. During the year does it rain the most in June. During winter months, in November, December and January are similar levels of percipitation being more snow or hail than rain.
The coldest months are December and January, November and February can be cold as well but are normally less colder. The hottest summer months are July and August, spring from March to June and autumn in September and October are nice in general. It can rain but it's not badly cold and normally it doesn't rain a lot.
For winter you should bring a warm jacket, some pair of nice boots and scarves, a weather jacket would be good as well. There is more wind than rain but the wind can be harsh. An umbrella is really useful as well. For summer you can wear anything you want, from short pants over sleeveless tops to skirts and dresses, not only for girls! :-D
-> put two charts of Giessen climate... https://de.wikipedia.org/wiki/Gie%C3%9Fen category: Klima -> title: Gießen Klimadiagramm; Monatliche Durchschnittstemperaturen und -niederschläge für Gießen
-> put chart of weather forecast in Giessen like in happyerasmus page... best take it from wetter.com