Hi Tom,
I have never seen any pattern in activity vs day of the week - it's totally random! (Apart from weekends of course - no flying at all then).
Weather wise, yes, it affects activity. Low visibililty (cloud, fog, heavy rain etc) will all put a stop to flying. As well as make the side of an exposed Welsh hillside quite an unpleasant place to be...
If you're in place by 8am you'd be very unlucky to miss a pass - I can't remember hearing of many (if any) movements before then. At some of the places (Bwlch & Corris esp) the car park is quite small so you need to be reasonably early to get a parking spot. On this score too 8am should be OK 99% of the time.
HTH!