renshuu: You will not use に, or any particle, for that matter, with ‘relative’ time phrases, like きょう [today], らいしゅう [next week], or せんげつ [last month], that change depending on where in time you are.
also renshuu: 今日は海に行きましょう！
I'm confused… renshuu uses "は" with "今日" that's a relative time phrases.
My condensed (and likely inaccurate) version would be, when we say "X is happening at time Y", if Y is a concrete time you can point at like "5:00" use に, but if Y is a "relative" time expressions like "yesterday" we use it like an adverb. Outside of that specific context, "relative" time expressions are nouns and follow normal rules.
