How do you plan for Boxing Day + Black Friday + Glastonbury peak load?

UK retail has three peak windows, not one, and Manchester fast-fashion has a fourth. (1) Black Friday (~£8bn weekend) bleeds into Cyber Monday. (2) Boxing Day is the UK's #1 retail day, Arndale + Trafford Centre footfall plus boohoo / PLT / Missguided online surges. (3) January sales for THG-style multi-brand. (4) Manchester-only: Glastonbury (Jun, festival-fashion DTC spikes 4-8× from Manchester fast-fashion brands), Manchester Pride (Aug, LGBTQ+ DTC), back-to-uni (Sep, 60k Manchester / MMU students), Premier League season (Aug, May, Man Utd / Man City merch DTC). We build a war-room: 60× synthetic load test with k6 or JMeter, Redis cluster + Varnish FPC smart-purge, queue-isolated checkout PHP-FPM pool, Adyen tokenisation for repeat buyers, read-replica DB for browsing, Fastly or Cloudflare in front. Pre-cutover load tested 2 weeks before peak.