With due respect to some posters above, the information about the "whole monorail system" being down during those daytime hours is not accurate. Recently the daytime closures, when applicable, have been on the Express Monorail line, not the entire system. There have been times over the last few years when the daytime closures were more pervasive across the lines (including the all-important Resort Line), but that hasn't really been the case recently.
The Times Guide posted above is your best bet for accurate info. But, as always monorail hours can/will change and refurb work can migrate around, so always check it when there to be sure.
FWIW I have stayed at all three monorail resorts at various times this past year (Feb, July, and Sept) and aside from a few mornings where it wasn't running quite as early as I wanted for a morning ADR, I experienced ZERO negative impacts from any refurb work (and we are heavy monorail users at all times of the day).