The sentence, "I would leave tomorrow", is grammatical, but is lacking some sense (subjunctive mood)
The Subjunctive Mood
A verb is in the subjunctive mood when it expresses a condition which is doubtful or not factual. It is most often found in a clause beginning with the word if. It is also found in clauses following a verb that expresses a doubt, a wish, regret, request, demand, or proposal.
Source: http://englishplus.com/grammar/00000031.htm
"If I would leave tomorrow..." (correct)
ALSO:
(1) I was going to leave tommorow. <- WRONG (tomorrow indicates a future time reference, you cannot use it considering the context you provided)
"I am going to leave tomorrow." (correct, the day before leaving)
"I left [yesterday]." (arrival)
(2) I was leaving tomorrow. <- WRONG (same as above)
"I will be leaving tomorrow." (correct, the day before leaving)
"I left [yesterday]." (arrival)
I will not discuss the use of past progressive here, but I'll provide a link http://www.englishpage.com/verbpage/pastcontinuous.html
*I left tomorrow. <- WRONG (same reason)
You cannot say, "I ate tomorrow" in the same way that you cannot say "I will eat yesterday".
per definition:
tomorrow
[tuh-mawr-oh, -mor-oh]
noun
1.
the day following today:
Tomorrow is supposed to be sunny.
2.
a future period or time:
the stars of tomorrow.
Since 'tomorrow' is a future time reference, it is wrong to use it in the above quoted sentences. The point here is you cannot simply use a future time reference in a sentence referring to a past action. Also, it is ungrammatical to say "I would leave tomorrow" (in the context the OP provided). 'Will' is a modal used to indicate future tense, hence the main verb's tense should be changed. As above explained, you may use it if you are indicating a subjunctive mood.
Remember that leave/leaves (present); left (PAST); and will leave(future).