Leave a bicycle in the rain for a day - is it safe? I use my bike (gt avalanche 1.0) to commute to work. Now it's autumn and so rain can happen just every day.

So the questions are: is leaving my bike outside harmful? Should take the seatpost with me or better leave it in place?
It's no problem, but there are a few things to consider:


Keep all the moving parts well oiled. Note that even if you only ride on roads there will be more dirt getting on it, so you'll need to give the chain a good clean, let it dry, and oil it.  This should stop the mechanicals getting too rusty, some screw heads will rust but not too badly.
Wet rims don't brake as well, especially if the wet isn't pure, so after riding off the first stop may take longer (and be noisy). If the rain has stop it may be worth applying the brakes (carefully) once or twice right after starting your ride, to get the rims dry.
Get a waterproof saddle cover, then take it off when you want to ride - so much more comfortable than a wet saddle, especially if the saddle has stitching which lets the water in.  This is better than a plastic bag, because you can leave it on, and only remove it if it's wet. Alternatively, get a saddle with a plastic surface, then you can just wipe off the rain.
If you're expecting really heavy rain, your lights might not take it (as I found out recently) so you might want to take them with you (of course you might want to anyway so they don't get stolen). This is for removable lights; if you have lights that are meant to be installed permanently (for dynamo use), they will gererally be watertight.


If you ride in the rain, then keep the bike in a shed on a damp day, it won't dry out properly for hours anyway, so the actual outside storage isn't much extra punishment on top of the ride (assuming of course that it's wet all day)