"The next day is Monday."

Translation: अगला दिन सोमवार है ।

[SainmitreP]

When do you use अगले and अगला?

[hptroll]

There is another aspect to it: in some cases, it is the oblique case although no postposition is present in the sentence: the postposition is implied. See these comments: https://forum.duolingo.com/comment/28369697
https://forum.duolingo.com/comment/29382275

[Anand309040]

Like अगला लड़का। अगले लड़के। अगली लडक़ी। अगली लड़कियों

[Anand309040]

अगला अगले अगली are used with singular masculine, plural masculine and feminine (singular and plural) words respectively

[JustusRobi3]

Mnemonic for सोमवार = Monday: सोमवार has a [m] sound in it, like English "Monday".

[another-dave]

But मंगलवार (Tuesday) also has an M sound & even begings with M! :)