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> **This question also has an answer here (in German):**  
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> *[Am Samstag, dem/den](https://german.stackexchange.com/questions/38932/am-samstag-dem-den)*
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> 

It's more logical to me to be "am Montag, dem 10. August" since without specifying Monday, I would say "am 10. August" Instead, it's like the article is taking on adjectival endings.