The Hub of the Day is generally a high quality hub that meets all of the criteria in the link renegadetory posted. It's written well, has at least one legally attributed photo, 500 words or more, and makes good use of the various capsules. It's also of general interest. Unless your hub about your summer trip had advice for the general public, it may not be of general interest (not to say that it wasn't a good hub, though!).
Since you just published it, it may not have been seen yet by those who choose the HOTD. Give it time; it is a beautiful hub, and it might catch someone's eye!