The logs are dated in UNIX-time format by default. You'd need to either involve the `date` command to convert to a 'regular' date format and narrow down which entries are from the date in question, or perhaps easier would be use one of the many tools and scripts I can see on the internet, for examining squid logs and/or converting them to a standard (e.g. httpd) log format, and then isolate the days you want.
More often than is probably healthy, I tend to do things in possibly the less-easy way,
so if I were in the situation you describe, I might be inclined to scan the file line by line with a script, converting the date of each line, and then grabbing out the lines whose dates correspond to the day in question, and dumping them into a new file..
If you want an example code of how I might do this, post a few lines of the log in question and sometime later I'll show you what I mean. But meanwhile, it's quite possible that someone who is familiar with squid-logs and does this sort of thing regularly, will offer a 'commonly used' alternative to what I propose.