Learning as I go here...
An excerpt from squid.conf under the ACL tag information...
# acl aclname time [day-abbrevs] [h1:m1-h2:m2]
# S - Sunday
# M - Monday
# T - Tuesday
# W - Wednesday
# H - Thursday
# F - Friday
# A - Saturday
# h1:m1 must be less than h2:m2
I believe this combined with http_access is what you're looking for.
So, if you were to add these line (in the appropriate area, probably by the rest of the acl/http_access tags, after any specific allow designations, is my guess):
acl 8to8http time MTWHF 20:00-8:00
http_access 8to8http deny
I think that might be exactly what you're looking for. This is assuming you only want access during business hours...but you can probably see that it'd be easy to add/remove days and adjust times given this information.
As I understand it: The acl is an "access list" tag that can be defined in a large number of ways, with the time method being what you need in this specific instance. The http_access tag is simply a tag that will take the form of: http_access <previously defined acl> allow|deny.
Again, I must state that I really don't know here, but in the time I was looking at documentation and at the squid.conf itself (this is incredibly well documented compared to most config files), but I think this is along the lines of what you need.