on tomorrow , tomorrow两樣講法，邊個正確?

The meeting will be held tomorrow. - Correct.
Even though "tomorrow" is a date and for a date we usually use "on" as preposition, we do not use preposition in this context.
There are exception that we need preposition for tomorrow :
I am getting my dress ironed for tomorrow.

On Tomorrow

This page clearly says that we say on Monday but not on tomorrow:

http://www.wsu.edu/~brians/errors/ontomorrow.html

2008-10-14 14:41:35 補充：
But it doesn't mean that "tomorrow" can never follow "on". So we can find titles like "Eye on tomorrow" and "Gambling on tomorrow". (but that's because "eye" and "gambling" require "on" to follow)

2008-10-14 14:50:40 補充：
regarding "by" vs "on" tomorrow, i guess all answerers are easily led to say that "wrong" way in explanation. so when telling the asker the difference, we might say ...

2008-10-14 14:50:44 補充：
"by" means on or before tomorrow, and "on" means exactly on tomorrow... you see! it's hard to avoid putting "on" and "by" together!

hi, THE MEETING WILL BE HELD TOMORROW is correct! If u want to use {on}, it should be the date after it; e.g. THE MEETING WILL BE HELD ON 10 OCTOBER 2008.

一定是第2句正確，就算要用都是+for