May through September are the most pleasant months, when outdoor cafes and beer gardens are open and busy. Rain is always a possibility, but not constant enough to thwart your plans. The numbers of tourists can be high.

May to March and October to early November also have plenty of mild and nice days, with the advantage that tourist crowds are somewhat smaller.

November till early March bring greyer skies and colder temperatures. Keep in mind that in the dead of winter there are only 6-8 hours of daylight and in December, the sun sets around 3:30pm. If you're not headed to a winter resort, the tourists are sparse.

Mid-February marks the end and climax of the carnival season, and since the Cologne carnival attracts lots of visitors, room rates can be considerably higher at this time. If you want to have a wide choice in accomodation, you should book well in advance for the week preceding Ash Wednesday. The same goes for the days around 11th November, when the cheerful madness begins at 11:11 am.