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Hi Simon, I agree with Chris comments. The way you're implementing the "seasonality" update is certainly not the most SEO friendly nor the easiest to implement (by replicating your entire structure in your main and a sub-domain, which is not the best from a user experience, architecture and crawling perspective, even if you manage to not fall into content duplication issues by using canonicals or noindex, follow meta robots). There are many websites that update their content based on the season, for example, take a look at how Zappos does it, by enabling many offers in the available spots of their home page, giving them enough visibility, that take the user towards a internal landing pages gathering the offers targeting that season (which are the ones that will rank for those season keywords and you can re-use each year) or directly to the categories of the type of products in offers. If you need to update your look and feel you can do it without touching the Website structure, by updating your menu, text, background colors through CSS and even enabling some additional design elements, that won't affect the site architecture. I hope this helps!