Maybe my experience isn't the norm, but I wouldn't leave shopping till the Saturday if it is the busiest day. We were there Tues-Fri the first week of December and found the whole time pretty even, with late morning and early afternoon having the longest, but still manageable queues. We pottered and browsed around the shops, planning on buying stuff the last evening - big mistake. On Friday evening the shops were like Claire's Accessories on a Saturday afternoon at home! In other words, you had to jostle and shuffle just to see what is on display. We ended up not buying half the things we'd planned to buy.
If I were you I'd buy as I go along, leaving Saturday as an atmosphere-soaking, people watching day, with a few opportune rides squeezed in if you happen to be in the right place at the right time.