23rd - 26th June. The Thursday is called the moving motor show and is more a trade day with manufacturers using the hill for demos of their latest products. It is the quietest of days and most of the static exhibits are easy to see due to the smaller crowds. You can often get free invites to this from manufacturers, Alfa often a=have an application on their website.
Fri, Sat & Sun are the days that the stars demo various machines on the hill. Crowds are bigger especially near the track and patience may be required to get to see what you want. Sun is by far the busiest day.
I like FOS it is a good day out but if you want to see the cars and stars in action but minimise crowds go on the Fri, if you just want to see the static displays go on Thursday