All the above is great advice! If, however, you want to tidy up your photos within a catalogue and clump them by subject, rather than date, the following is fairly easy:
1) Make sure your "quick collection" is empty.
2) Plan a subject hierarchy, which would have overall topics like "clients", "events", "nature", "landscape" and so on. It becomes logical to have subfolders under the main topic (different clients, events, places, whatever). Use the plus sign on the folders pane to make new folders with these main names. It is probably easier to worry about subfolders later.
3) Decide on your first topic. Work through your photo collection from the beginning (or in areas where associated images lurk). When you find an image that will go in that group, hit the "b" key. You will save a lot of time by using the command or control key or shift key to select multiple images. Doing this puts your images in the quick collection. To make the process easier, you can first sort your library by date, which will make initial selection of images very quick, as they will have been taken at the same time.
4) When you have put all the images from a particular topic in the quick collection, open the quick collection. Hit "command key plus A" (control key plus A in Windows)
to select all the photos.
5) Right click on the destination folder. Use the option to "move selected images to this folder", or something similar.
6) While all photos in the quick collection are still selected, hit b again. This will clear the quick collection and allow you to start again using a different topic.
7) When you have done this on a crude level, you can open any folder and make new subfolders, either dragging images straight in or following this process again.
It might sound long-winded but is safe and easy and actually quite quick. I have used something similar to move photos around various hard drives and it's great. It's also a very good technique for discovering surplus copies of images. If you do this over several catalogues, you may discover your topics are the same in different catalogues; that you have used one catalogue to shift images that now appear as "missing" in another catalogue or that you have lots of duplicates. Either way, it will tidy your collection.
Finally, whether or not you do this, lightroom metatdata browser lets you call up photos under different dates anyway. Maybe that's all you really need?
Don.