How do you spell 'when is' in a contraction?

Answer 1

The spelling is when's as in When's the party starting?

Contractions that merge a pronoun with "is" replace the I with an apostrophe.

(Some styles consider this informal language.)

Examples:

Where's the fire, buddy?

When's the big day?

Who's the boss around here?

However, the contraction Who's (who is) is never used as a possessive. There is a separate possessive form whose. (e.g. Whose car is this?)