In carnival (which is BIG in this part of Germany!!) people in Düsseldorf don't shout 'Alaaf' (that's in Cologne!) but 'Helau' ...
Be prepared to get drunk and dirty when going into Düsseldorf's old town on carnival days. There is thousands of people there, all drunk, many dressed up. There will be music you don't hear every day and the whole town is one big party!
Altweiber is said to be the best day to celebrate. It's women's carnival and the women rule the town that day. They take over the town hall and they are allowed to cut ties of all men around! Of course it doesn't mean that there's only women going out that day - how boring would that be? ;-)
Personally I liked the atmosphere around the Uerige pub best. The music was good and people were friendly. On Ratinger Strasse, one of the busiest streets of the old town, it's too crowded and there's no atmosphere somehow...
The area around Düsseldorf and Cologne is the only area in Germany where you can find the "real" carnival - street-carnival.
It starts on "Altweiber", the Thursday before "Rosenmontag" and "Veilchendienstag". On this day women take on masks and costumes to look like old women. And they are allowed to cut off the mens ties.
On Sunday are the "Veedelszöch", the carnival-processions of the quarters. The big processions of Düsseldorf and Cologne are on Monday, Mönchengladbachs "Veilchendienstagszug" is on Tuesday. The participants of the carnival-processions throw candies and other things for the people along the streets. Especially in Cologne nearly everyone wear a costume, but also in Düsseldorf and Mönchengladbach you should at least wear a special make-up.
Karnival is in February, but the exact date is always different.
One of the most important events in Düsseldorf's social calendar is the Karneval-weekend which takes place around the end of February with a colourful parade on Monday. While for most people it's the highlight of the year I have to admit that I always temporarily emigrated somewhere else for that weekend. Maybe I just was not drunk enough on the few occasions that I did hang around, but I did not like the way that for five days the city was full of drunks trying to chat you up.
Around Karneval (Carnival) Düsseldorf gets crazy: people dress up, get drunk, and party. The typical music is pretty stupid. There is a big parade on carnival´s monday (Rosenmontag), as on the picture. Carnival is usually in february, or might be at the beginning of march.