The best way to create your search strings is step by step.
in the search string the AND is always implied unless you say something different, so no need for that.
Try this first without any date_hour restriction:
date_wday!="sunday" date_wday!="saturday"
You will see it will work because you are telling Splunk to search all the fields but the ones that have the value Sunday or Saturday
For the time restriction, I'd recommend you used the eval function. http://docs.splunk.com/Documentation/Splunk/5.0/searchreference/eval
anyhow, you can always have this working
date_hour>=6 date_hour<=18 date_wday!="sunday" date_wday!="saturday"
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