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. During that time the cable car over the river Rhine will not operate. From the end of November until the 23rd December the Christmas Markets will be open as well and attracts a lot of tourists to Cologne. There are currently six markets in Cologne (Old Town with a ice rink, next to the Cathedral, Neumarkt, Rudolfplatz, near the Chocolate museum and in Muelheim). They are usualy the busiest over the weekend.

February or March marks the end and climax of the carnival season - starting on Weiberfastnacht on a Thursday and finishes on Ash Wednesday. During that week of street carnival events on the streets, pubs and tents happen throughout Cologne, with the highlights being the parades on the Sunday and the main event being the Rosenmontagzug on Monday, when  usually over 1.5 million people watching the parade on the street. Since 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.