Depends if DJ Harvey is back! Last year, Harvey's final date was Monday 3rd Oct, so you could be there for the finale. Otherwise, Mondays are quieter and usually just with pool-side DJs on during the day, until around 11pm.
Weekends tend to be busier and with more events on in the evenings (Friday to Sunday and sometimes Thursday have events on inside at Freddie's). However, I love the daytime pool sessions during the week, and find them more chilled. But the daytime Sundays are busy, if that's for you're looking for.
Also, if you're staying at Pikes as a guest, you get one of the poolside / garden beds, which are reserved for hotel guests, which is ideal when it's busier.
Pikes is always a good idea!