baş kün

/ baʃ kyn /

Cases

SingularPluralWhen to use it
nominative baş kün the Monday baş künle the Mondays the plain dictionary form — the doer of the sentence
genitive baş künnü the Monday's baş künleni the Mondays' the owner — whose it is
accusative baş künnü the Monday (as the object) baş künleni the Mondays (as the object) a specific, known object — whom or what
dative baş künge to the Monday baş künlege to the Mondays direction or recipient — to where, to whom
locative baş künde at / in the Monday baş künlede at / in the Mondays where it is; also having — mende bar, "I have it"
ablative baş künden from the Monday baş künleden from the Mondays where it comes from; also "than" when comparing

The endings are the same ones the pronouns wear — that page explains when each case is used.

With an owner

One ownedSeveral owned
my baş künüm my Monday baş künlerim my Mondays
your baş kününg your Monday baş künlering your Mondays
his / her baş künü his / her Monday baş künleri his / her Mondays
our baş künübüz our Monday baş künleribiz our Mondays
you all's baş künügüz you all's Monday baş künlerigiz you all's Mondays
their baş künleri their Monday baş künleri their Mondays

The overlap in the last rows is real, not a mistake: baş künleri can mean "his / her Mondays", "their Monday", or "their Mondays" — context decides.