What does the term "a free concert in the mud" mean in this sentence?

Solution 1:

The concert in question is the 1969 Woodstock concert. It became a symbol of the changes in western society during the 1960's.

Wikipedia:

It is widely regarded as a pivotal moment in popular music history, as well as the definitive nexus for the larger counterculture generation.

At some point, because of the overwhelming number of people trying to get in, the organisation announced "this is now a free concert" - the words became kind of famous.

Because of the rainy weather, the whole thing was a pretty muddy affair.

Solution 2:

A 'free concert in the mud' is an example of an oblique reference: a turn of phrase where the speaker indirectly describes some specific object as if it were a class of objects - with the idea that the intended audience will realise based on context that it actually refers to the specific object.

So in this case, the author refers to 'free concerts in the mud' as a class of generation-shaping event. However, to the intended audience - people familiar with western popular music history - that class can describe only one thing: the 1969 Woodstock Concert.