I don't do that personally. If anything I'll create a page dedicated to the sale/season (such as Black Friday) and then have a popup or something that informs them. I may alter my titles around some when trying to optimize early on, but I don't do much with them once I've started ranking for my desired keyphrases on a particular page.
Some keywords have way too much competition so generally I don't target the same ones that Amazon etc do.
Also personally I wouldn't change the keywords monthly..might be frowned upon by the search engines (spam site..) also not a good idea should I want to develop the domain or sell it because of the "indecisive history". Lot of buyers check out sites like https://archive.org/web/