If your house Plumbing is put together right rainwater won't go into the septic system. A properly designed septic system only takes "waste water". Story water is supposed to drain away from the house or into a "sump".
If you have a septic tank it needs to be cleaned out professionally. If you hva e septic field it needs to be enlarged.
Yes, the ground can become saturated and the laterals have no place to drain to.
The answer would be a thunderstorm.
Heavy Rain happened in 2010.
Thunderstorm
Heavy Rain was created on 2010-02-18.
A strong wind that can bring heavy rain
drizzle is very light rain and well heavy rain, is well, heavy
One day of rainfall would not fill an empty septic tank.
Heavy continuous rain is lots of rain coming down that lasts for a long time.
A downsliding of earth after heavy rain is called landslide.
Heavy rain can can last any resonable amount of time.