The question When should sales spike during fourth quarter Sales? cannot be answered with a simple blanket answer because each merchant, and their product mix are going to affect sales, but if I were to take an average spike period, for the bulk of online retailers you begin to see 4th quarter Halloween traffic spiking October 14th, through the 30th. Another smaller spike occurs during the two weeks after Halloween as merchants closeout overstock and clearance items.
Christmas begins to show an increase as early as August of each year as some of the sales are early shoppers looking for seasonal variety in their clothing purchases. By Christmas, stores are only stocked with Winter apparel.
The traditional Christmas rush is trackable online similar to the brick and mortar stores with a big spike beginning near Thanksgiving weekend, and continuing through about the 12th of December and tapering down to Christmas Eve. Sales online after the holidays show a much smaller, but consistent spike throughout the first two weeks of January.
If you’re waiting until the fourth quarter each year to plan your companies ad spend, you way too late. Budgets, SEO work and the like should be done well ahead of the fourth quarter, usually no later than June or July of each year.