Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
‡ septĭmātrūs, ŭum, f. [septimus], a festival celebrated seven days after the ides, acc. to Varr. L. L. 6, § 14 Müll.; and Fest. s. v. quinquatrus, pp. 255 and 257 ib.