Russian Wikipedia has an article on: <span class="searchmatch">Петров</span> <span class="searchmatch">день</span> Wikipedia ru IPA(key): [pʲɪˈtrov dʲenʲ] <span class="searchmatch">Петро́в</span> <span class="searchmatch">день</span> • (Petróv denʹ) m inan (genitive Петро́ва дня́) Saint...
See also: <span class="searchmatch">Петров</span> Пётр (Pjotr) + -о́в (-óv) IPA(key): [pʲɪˈtrof] <span class="searchmatch">петро́в</span> • (petróv) (possessive) Peter's (in collocations) <span class="searchmatch">петро́в</span> <span class="searchmatch">день</span> ― petróv denʹ...
it is written capitalized as <span class="searchmatch">Петро́в</span> (Petróv) and has the normal declension for these types of adjectives. <span class="searchmatch">петро́в</span> <span class="searchmatch">день</span> ― petróv denʹ ― Feast of Saints...