The general rule of thumb is you don't want to change URLs. Changing URLs means links pointing to those pages and bookmarks referencing those pages can break, leading people to an error. From an SEO standpoint, you also lose any authority assigned to that particular URL (like links referencing that URL) if you suddenly remove that URL from your website.
You can get around that with redirects. Typically, though, for best SEO and simpler management, you want those redirects to be permanent (a 301 redirect). In your case it doesn't sound like permanent redirects would make sense as you need to continually have a different URL. You could use temporary redirects as you change the URL, but that could become a management nightmare.
So, what pages do you need - one URL or three for each type of sale? Well, a couple questions for you here:
- Do these sales overlap or do they reach run at separate times of the year?
- Are the sales being targeted to the same general audience (should the same people see all three sales)?
If the answer to all three is yes, and my guess is that the answers probably are yes, then I'd think you can go with a single sale page. The content of that page changes during the year as the sale changes. That way people know to bookmark and return to this page to see the latest sales.
If the sales overlap and you can't have the same page talk about multiple sales or if the sales are targeted at different groups of people, then maybe you do need to have a separate page for each. In that case, you would want to keep the URL active for each sale, without changing the URL. Instead, you would just change which sale is linked to from elsewhere on your site when a particular sale is active. The inactive sales would still be pages on your site, with a consistent URL, but would not be heavily promoted when the sale is inactive.
The first option, with a single page, is certainly a cleaner way to go and hopefully that makes sense given the audience targeted.
I hope that helps!
