Figured this out...here is what to do if anyone else is interested in using this as a recipe...

$r = array(); // an empty array
if ( MM_DAY_OF_WEEK == 1 ) $r[] = 150;
if ( MM_DAY_OF_WEEK == 2 ) $r[] = 151;
if ( MM_DAY_OF_WEEK == 3 ) $r[] = 152;
if ( MM_DAY_OF_WEEK == 4 ) $r[] = 153;
if ( MM_DAY_OF_WEEK == 5 ) $r[] = 154;
if ( MM_DAY_OF_WEEK == 6 ) $r[] = 155;
if ( MM_DAY_OF_WEEK == 7 ) $r[] = 156;
return $r; // may be empty, or contain any module id.

OK, now something is definitely wrong somewhere. The expected modules suddenly stopped displaying. When I run metamod in debug mod, it is no longer showing any included modules even though it worked fine yesterday.

Here is the debug output
MetaMod debug info:
Module ID: 151
$option: com_content
$view: frontpage
$id:
$Itemid: 1
$timezone: America/Chicago
$language: en-us
$language_code: en
$language_region: us
Start date/time has been reached.
End date/time has not passed.
Including modules:

That definitely was it. The module for Monday is showing today. Thanks

BTW: For the last else (return) line, how do you add the module if all conditions are not met in the array?

So, for example, if I want to display module 200 and 201if conditions in the array are not met, is it...
return $r; = 200, 201//
or
return 200, 201?

thanks