The good news is that the septic system is probably working corectly. Make sure you have the tank pumped out every two years.
I had the same problem with a home I had in Central Wisconsin. The morning before my daughters high school graduation party, the gentle north breeze on the beautiful clear day was carrying the septic fumes down over our deck. the smell was not pleasant. (to say the least)
I took a couple of plastic bags and covered the vent pipes at the far end of the septic field. this stopped the fumes from comming out of the vent pipes on the roof. After the party, I removed the plastic bags, and everything returned to "normal".
I guess the septic smell is part of the price we pay for living in the country!