First, open iPhoto, and then select a source from the sidebar to search within.
You can pick “Events,” for example, if you want to look through your entire library, or you could choose something like “Last 12 Months” from the sidebar to narrow your search to that subset of your images. Anyway, after you’ve done that, select the search button at the bottom of the iPhoto window…
…and then click on the drop-down arrow within it. That’ll reveal a pop-up menu with all of the parameters you can use to do searches.
Pick “Date” as I’ve done above, and a new pop-up will appear.
See how January, February, and March all have bold text? That's how iPhoto'll tell you that it has images that match those dates. Cool! And from there, you can click on any month to view its associated pictures, or you can hold down Command and click to select multiple months.
Use the arrow buttons at the upper-right corner to switch years, and if you'd like to select an entire year at once, just tap the year's number at the top.
Finally, if you click on the arrow in the upper-left corner, you'll toggle to a month view instead, which'll let you pick specific days to search for (and again, you can hold down Command and click on multiple days if you need to).
As you can see, this view works the same way that the year view did in that days that have associated images will be in bold, so you can tell when your photos were taken and pick from existing items to find precisely what you're looking for. The only caveat here is that sometimes the bold thing is glitchy; if you find that yours isn't showing you which dates you've taken photos on, either quit iPhoto and reopen it or try clicking the “x” in the search box to clear out your search and then reselect your source from the sidebar.
Even with that caveat, this feature seems indispensable once you know about it, but it's not incredibly obvious that you can search this way. Sometimes I believe Apple just likes making us poke around to find stuff.