Auto word change in documents

I need a search engine friendly script that will change words automatically in my articles. Example: The best selling BLUE Lights for AUGUST The best selling lights this month are BLUE lights. We like BLUE Lights because they provide a tremendous amount of light. Ok so there are 3 sentences above. We want a script that is search engine friendly or HTML code whatever is needed and search engine friendly to be able to change certian words in above 3 sentences. Example #1: August change to September Automatically on September 1st and then to October on October 1st and so on. Example #2: The word Blue to change to whatever color we specify on the 1st of Septmeber and again to specified word on Ocotber 1st and so on. We just need the code so we can put it on numerous articles and product descriptions. You must show me a working example on an exisiting website.