as Reyhan said. Yesterday=Dün, Tomorrow=Yarın,Today=Bugün. Day=Gün . Günün means Todays -ün suffix provides possession , as it is (apostrophe) 's in English. Todays=Bugünün . Notice -ün addition. Gün is day and Day's(I know its ridiculous but the logic is different among languages) is Günün. Hope helps

Günün şakası = Joke of the day Dün means Yesterday Nice day