NoVaCruiser60

New to Celebrity, doing the transatlantic route on Silhouette in April. I see an offering to buy a 10 or 13 day Spa pass. How does a 10 day pass work on a 13 day cruise? Does it "start" the first time I use it, or the first day of the cruise? Does it run for 10 consecutive days, or is it for ten days of use (non-consecutive)? I can see getting the cheaper alternative (I don't need to access the spa EVERY day), but it all depends upon how flexible the pass is. Any ideas?