What are the optimal conditions to install a hardwood floor?
There isn't a right time.  In the summer the wood will expand and in the winter it will shrink.

The biggest question is whether there are large temperature changes where the wood will be installed.  If the house is always conditioned then just make sure it is in those conditions or warmer when installing the wood.  

My personal opinion is that I would make sure that your hardwood is warmer/more humid than normal.  And it takes a few days for wood to totally temperature/moisture adjust.  Don't sit your wood outside.  If you have the heat on sit in next to a vent.  If the air is on - away from the colder air.

Slight shrinkage is basically nothing. Might notice some gapping but you won't notice unless you are on your hands and knees.  A lot of expansion can lead to buckling though and that is the last thing you want to deal with.  I installed mine at the end of last spring on purpose last year.