Can Lightroom sort photos from several days by time of day instead of date?

While I can't think of a way to do it with the built-in functions of Lightroom, it can be done with the help of the LR/Transporter Lightroom Plug-in (donationware; free version restricted to modifying 5 images at a time). If you don't choose to donate, you'll need to repeat the steps 2…4 below for every 5 images until you're through your whole collection.

We'll use this plug-in to assign the capture time of each photo to the "label" field in its metadata, which will serve as a scratchpad for this purpose. We will rearrange the capture time to the time/date format HHMMSS YYYYMMDD and then make Lightroom sort all photos by the "label" value. The effect being that the capture time (and foremost, the hour of day) will be the primary sort key, as desired.

1. Once you've downloaded and installed LR/Transporter in Lightroom, switch to Lightroom's Library view and select your entire batch of holiday photos (or the first 5 images, see previous paragraph).

2. Select Library > Plug-in Extras > Change Metadata using LR/Transporter…

3. In the "Change Image Metadata" dialog box that appears, select label from the left drop-down menu. In the textbox to its right, enter this code:

   {dateTimeOriginal[%H]}{dateTimeOriginal[%M]}{dateTimeOriginal[%S]} {dateTimeOriginal[%Y]}{dateTimeOriginal[%m]}{dateTimeOriginal[%d]}

4. Click the OK button. An "Undo Warning" will appear; click the Continue button to confirm.

5. Now, still in Library view, go to the Sort Criteria menu at the bottom of the Grid view. Select Label Text from the dropdown list.

Voilà – your photos should appear sorted by hour of day.