Anytime the day before, from after noon until midnight (or later, depending on location and
preference), you may say, "
What are you doing tomorning?" --What are you doing tomorrow morning?-- or "Hey, can I
catch a ride to work with you tomorning?" --Hey, can I catch a ride to work with you tomorrow morning?--
During the wee hours of the morning, you can say, "I am going to school tomorning." --I am going to school (later) this morning.-- However, this is not a commonly used form of the word.