I say it should be at the end of the day, as this helps you write the whole day experience. In my opinion, I suggest you write better at night at the end of the day. Keeping a diary in the morning is different from keeping a diary at night. When you write in the morning, you can impact your mind throughout the day because the day hasn't started yet.
I started keeping a diary in my bujo to keep track of what I ate and the symptoms, to try to figure out what was triggering migraines. As such, I write a diary in the morning to set goals, etc. for the day, write a little bit at lunchtime to check in, and then write in the diary at night as a way to summarize the day and relax and review. In a way, I prefer a physical diary just because of the fact that I do so much on my computer that it's nice to spend some time out and focus on something else for once.
This is because when you work on these pages (or spend time “writing freely” as Mark Levy discusses in his book Accidental Genius), you are not setting out to write a diary. Whether you decide that it is better to keep a diary at night than in the morning, the act of keeping a diary of your thoughts is very beneficial. I made the mistake of buying online magazines before only to find out that I didn't “feel” right. This is a great way to keep a diary if you're struggling with depression or going through a very difficult time.
This could be useful if you're trying to eat better or just need something to start the process of keeping a diary. Keeping a conscious diary means spending time in the morning freely writing at least three pages of thought. And there is a surprising amount of research that supports the idea that keeping a diary at night is better than keeping a diary in the morning. Using a pen that you really want to use will ensure that you really want to write in your journal.
Author Ryder Carroll developed this method of keeping a diary, due to learning problems, as an alternative way to stay focused and productive. Bullet logging is a method to organize your thoughts, to-do lists, reminders, and future events by tracking them throughout the day. Although I have also tried to reduce the time by doing the journaling process in my favorite text editor, I have found that writing is too fast. While I don't write this journal this way often, when I travel it seems to me that keeping an interstitial journal helps.
If you are going to write a diary in the morning, set aside a realistic amount of time it will take to do so.