Car rocks side to side in idle and slow speed

is this dangerous? just started: subaru outback 2003- cant get it looked at until friday august 15

If the entire car is rocking while the engine is idling, that is…not good.
Subaru engines, by their nature, are very well-balanced, so this is definitely abnormal.
You may have broken motor mounts and/or the engine may be very badly misfiring.

Is the Check Engine Light lit up?
And, if so–is it lit steadily, or is it blinking?

Could be a lot of things, including the harmonic damper.
Hopefully it’ll just be a bad spark plug. When was the last time you had a tuneup? How well has it been maintained? How many miles are on it?