Not only do the buffs from the midsummer food stack with your normal raid food, but they stack with each other. So, for example, all your casters can be walking around with +44 dmg, +30 haste, and +20 hit.

And the “Festival Fortitude” 30 stamina buff that you get by being in a city when someone does a city quest stacks with everything and, insult to injury….: persists through death.

5 flowers get you 5 of a food. Putting out an opposite-faction bonfire nets you 10 flowers, your faction is 5.

Here’s a list of the bonfires: WoWWiki: Midsummer Festival

Elderberry Pie
Fire-toasted Bun
Midsummer Sausage
Toasted Smorc

Source: Stratfu