If you can't come on the day of your reservation, please let us know by the day before.
Literal
Reservation-of day [on-に] come-cannot case [topic-は], previous-day by [for-までに] definitely contact-[polite-てください].
Note the ら抜き contraction in 来れない — strictly, the potential of 来る should be 来られない (こられない), but 来れない (これない) has become near-universal in everyday speech and is now widely accepted. 場合は ('in the case of') fronts the conditional as a topic; までに ('by [deadline]') is the 'no later than' counterpart to まで ('until'). A concise example of how Japanese stacks time references.