Thank goodness. You did not miss anything. Traditionally it is believed that Varalaxmi/Varalakshmi Vrath can be performed on any of the Fridays in the month of Sravana as per Hindu Calander. Therefore you have 31 of Aug as well as 7th September still in your hand. Please go ahead and perform religiously. By the way you cannot find all the details of th epooja on the same site. I came across http://www.harsamay.com which has complete details of the pooja vidhi. These guys are so thoughtful of interpreting the SANKALPAM for American Conditions too. On top of it, they have Englist version, Telugu Version and Sanskrit version of pooja Sankalpam. All the best.