when better replace carpet in the basement
Around Xmas and January.  Generally new carpet lines go out in the spring.  So in December and January we have found that we can get carpet much cheaper and the installers aren't busy at all.  

Also don't let them glue down your pad or carpet.  You need to be able to move your carpet if you have a water problem.  (good installers would never think about glueing down in a basement but I see it all the time)