Do I need to reserved space in the tents to get seats during the week?

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Judy... I've been visiting during the Stuttgart festival. It seems that only groups can reserve seats in the tents. I think this is the same in Munich. We are planning to visit during the day and then go from there.

oops.. meant WE visiting ... Stuttgart festival... seats

For Munich Oktoberfest, during the day there's no problem finding a seat in one of the tents. It will be difficult at night, you can't reserve in advance, and they only serve you if you are seated. You just have to wait around, as people come and go all the time, and grab a seat.

What are the dates for Oktoberfest?

It's always the two weeks ending on the first Sunday in October. So this year it's Saturday Sept 18 to Sunday Oct 3.

There is an Oktoberfest website (English version available):
http://www.oktoberfest.de/
BTW, Munich natives will say "Wies'n" (pronounced weeesn) instead of Oktoberfest, literally meaning meadow as the fairground once was green... Did you know that the Oktoberfest dates back to 1810? Actually the first Oktoberfest was a festival on the occasion of the marriage of later Ludwig I. (not to be confused with Ludwig II. who built Neuschwanstein).