I don't get it....

I've been watering my seedling flats (veggies, annuals & perennials) about every 5 days or so, each flat on an as-needed basis. Basically I wait until the soil is pretty much dry all the way through in each plant in each flat before watering them (from underneath) for about an hour.

Yet today, after being away for 5 days I was surprised to see a single fungus gnat emerging from the growing mix as I was spacing out my nicely growing seedlings and culling some in the process.

1) Is it because my basement has no ventilation?

2) What is a quick way to get this under control ASAP...I have a party to host this weekend and there is no way these buggers are ruining my party. Last year when I grew my seedlings in my garage they congregated to the center of my kitchen ceiling for some reason. But that didn't happen until early May.

3) Is there a preventative measure I could have taken other than keeping the watering of my seedlings to a minimum (which I did)?