4
modules/system/templates/system_siteclosed.html
alternatively
themes/your_theme/modules/system/system_siteclosed.html
When you edit templates first make a backup or original file so you will have a fast rollback just in case. Go in preferences/general settings in admin and activate the direct check of templates for modifications so you won't loose time emptying cache by hand via ftp
if you're used to smarty and XOOPS then you'll know your way around the file, if you're only used to HTML then the stuff you don't understand what they mean are smarty and most of them are self explained such
<{$lang_siteclosemsg}>
which obviously reads as "site closed message lang constant" or something like that with the same meaning :)
good luck