- for the long term
Every day is served in a separate text file. The calendar allows you to easily browse the files.
It needs Microsoft .NET Framework 3.5 or better, and it is free.
I am not sure that I would use this program as a journal or diary on a daily basis, even if I were to restrict myself to plain text. I would probably prefer a NotetabPro outline file, with a separate heading for each day. But Notetab versions that do outlines are not free.
You could do a lot worse than this program! I recommend it. If you need a simple program for keeping a plain-text journal, this one should fit the bill.