As Ben says, Arcalis is often busy at the weekend as locals go there, plus lots of holidaymakers go there on the Saturday because it's often their only day without lessons.
elcauben
Arcalis does tend to be super busy with locals at the weekends. Weekdays are much quieter and are usually a better option. I´d recommend people start off in Arinsal, especially on a Sunday when it tends to be super quiet. Move onto Pal and Arcalis a day or two later. That said, if there is a fresh snowfall head over to Arcalis regardless as it has arguably the best offpiste in the pyrenees.
We'll be arriving on a Friday and will be snowboarding on a Saturday.... should we hop on the bus to the Arcalis sector straight away on the first day? or will it get really busy on the Saturday?