A word that means "at an unusual time or season"

Consider untimely

inappropriate to the occasion, time, or season ⇒ his joking at the funeral was most untimely

Also ill-timed

occurring at or planned for an unsuitable time

Note that both of these terms have somewhat negative connotations.

Are you looking for unseasonal? As in unseasonal snow?

un·sea·son·al

not characteristic or typical of a particular season; unseasonable:
unseasonal April snows.

This will not really work in both cases you used as examples since it assumes that there is an actual season for whatever activity you are referring to. Pancakes have no season as such, but decorations do.

You might get away with calling pancakes an incongruous dinner.

I like your "anachronism" (or "anachronous") for the examples you described but for an alternative, you can also say that they are "out of sync". The phrase can be used this way:

"Pancakes for dinner is out of sync with McDonald's menu and unfortunately not available this time of evening."

or it can be used as an adjective, like in this example:

"Don't you think you should remove your Christmas decorations before hiding the Easter eggs? Or are you going for a totally "out-of-sync" holiday theme?"