Frankfurt doesn’t exactly have a “best time” to visit. Being a hub for travel it sees tourism almost throughout the year.
The weather is fairly favorable throughout the year too. There will be showers in summer but other than that the atmosphere is always quite mild and pleasant.
Summer: Summer, as said before sees some rains and usually lasts from May to September. August is probably the hottest months.
Winter: From November the temperatures are colder but still quite mild in Frankfurt. It is sure to be a treat for people who want to see colder weather without getting frostbite.
Fringe Seasons: October and From March to April are also great seasons to travel to Frankfurt. The Weather is great and most importantly it does not rain as often as in summer. You can enjoy the mild weather while you see the sights.