いつ

いつ means "when". It sits where the time word would sit in the answer, and the question ends in か.

To ask when something is, name it with は and finish with いつですか.

No に after いつ

Time words like きょう and あした take no particle, and いつ behaves the same way: ×いつに行きますか. This holds even when the answer is a time that does take に.

The に in いつにしますか ("when shall we make it?") belongs to にする, "decide on", and is fine.

いつから and いつまで

から and まで do attach to いつ, for "since when" and "until when".

Usage notes

These go beyond asking a question.

いつも, いつか, いつでも

With も, か or でも on the end, いつ stops being a question word: いつも is "always", いつか is "sometime" or "someday", いつでも is "any time".

Other question words

なん何 (what), だれ (who), どこ (where), どう (how), いくら (how much) and いくつ (how many) sit where the answer will sit; 何, だれ and どこ usually take a particle as well.