Gontran quit on Thursday the 12th of March.

merve b - 

From the lesson -

when giving a whole date (day/number/month/[year]), such as :

Michaël a gagné au loto le jeudi douze juin.

Michaël won the lottery on Thursday, the twelfth of June.

 Le mardi 5 mars, j'ai rencontré Lola.

On Tuesday the 5th of March, I met Lola. 

Le monde a changé le dimanche 3 septembre 1939.

The world changed on Sunday the 3rd of September 1939. 

See also 

Expressing dates in French

 If you are asking why the article ( always ' le ' ) is used when giving whole dates, the answer is 'because it is' ( ' why ' questions on specifics of grammar rules are usually unhelpful, as grammar does not have to be logical, or consistent across all situations )