There are a number of ways to view archived strips, but here is one popular way: Click on "Find Comics" at the top of the page. Next, click on "A to Z listing" (or one of the more specific categories), and then click on the comic for which you want to view the archive.
Once the comic loads, look below the comic for the calendar navigation buttons:
You can either click the small calendar icon to open up a calendar, or you can use the "left" and "right" arrows to go back or forward one day at a time.
If you're looking for a specific comic but don't know the title or the date, you can visit AMU Reprints, which has a more robust search tool. Click HERE to go to the AMU Reprints site.