:There are now four references, including Britannica, giving the correct date, which is getting silly. You complain in your edit summary that you can't find them, so I've added quotes to two, to help you out. After that you're on your own: WP:SOURCEACCESS has more on this. I'm happy to concede that Britannica, alone, mentions the date you offer (but without explaining the context), so that's one of the sources I've expanded with a quote specifically mentioning 23 August.  

 :Russian Wikipedia has: "23 августа силы 14-го танкового корпуса армии Паулюса вышли к Волге севернее Сталинграда"  ("August 23 forces of the 14th Tank Corps, Army [of] Paulus, reached the Volga north of Stalingrad"), cited from the war diary of Colonel-General Franz Haider.  

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