I went to Harris & Lewis during the first week of October a few years ago, and we had glorious, hot, sunny weather -we even went in the sea, although the water was cold. The beaches are to die for, absolutely stunning.
I bought Harris tweed at the big shop by the ferry port in Tarbert, and also two pieces from a lovely weaver whose shed we visited - all still maturing nicely in my stash! In the big shop the pieces are all labelled with the weaver's name, as that's how they know who to pay when it is sold.
It was wonderful holiday, I even saw an otter come out of the loch, I hope you have a great time!