Well, actually nothing special at all!

In the 19th century, the famous German historian Johann Gustav Droysen (1808 - 1884) divided primary sources in "Tradition" and "Überrest" (tradition and relic). Even though the concept of Droysen is old and kinda outdated, it is still useful to explain why a diary should not be written for historians. "Tradition" includes everything people wanted to stand the test of time on purpose, like speeches, older historiography and memoirs. "Relics", on the other side, are typically private diaries, records, or even shopping lists - everything people just wrote down without having in mind that historians may find and discuss about it in 200 years.

Both is interesting to historians for different reasons - diaries are a fantastic source just because of the private point of view. The dutch historian Jaques Presser (1899 - 1970) called them "ego-documents": the author is writing and describing at the same time, nobody else. For cultural history, a diary can be a gold mine if the author had no intentions.

So to a lot of contemporary historical theories (like history of imaginations, history of ideas etc.), unintended tradition is very important, so keep it unintended. If you try to write a diary so future historians can work with it, they probably wont as much for source critical reasons.

I disagree with /u/IncubusFenris pretty strongly. There is no such thing as writing with no intentions. Writing for an audience of you does not free your writing from reasons for writing or hopes for its future.

When I was 8, I tried keeping a diary because some of the book characters that I liked kept them. When I was 12, I tried keeping a diary because I was starting a new school and thought it would be good to keep a record. When I was 17, I tried keeping a diary because my life was going to sh*t and I needed an outlet. These are all different intentions for writing that I desperately hoped would never go beyond me, and they all (or at least, the second two) very consciously shaped what I wrote down and didn't.

A source is not given value because it follows an exact set of rules. As a medievalist, I can promise you: a source is given value because it exists. (Both in the surface meaning of 'anything has value', and in the sense of, 'okay, now let's examine how this source survived until today and the path by which I came to be able to possess it.')

You're keeping a record of 2020 in hopes that people might use it as a historical source someday. That's awesome. It's also an intention of your writing. It just is. That's not a problem. I mean, it might be nice if you wrote that particular detail down or took a screen shot of this post and either printed it out and pasted it into a paper diary, or c-p'ed it to your electronic files.

...But as a medievalist, again, I can tell you that future-me would also be analyzing the snot out of your decision to do so. ;) (When you don't have much, you learn to work with what you've got. And these skills have been very effective when I have applied them to sources from other eras.)

All that said, one of the main sources I work with, in my own research, might be seen as sort of a diary (LOL). I would like to have the author's name specifically on it, and I wish there were dates on every entry. XD I want to know how old she was/when she was born. We have a good sense of her socio-economic and geographic location, which is very useful. If you're talking about a conversation you had, please say whether it was in person, over Twitter DM, reddit thread, via divine apparition by the Virgin Mary, &c.

But to historian future-me, it doesn't matter whether you decide to spend one day describing your bedroom or not; whether you decide to spend one day c-p'ing all the memes you see on Twitter (if you're keeping an electronic file or blog), and so forth. These are all things that I can work with. Just practically speaking, we have A LOT of information from today about sort of 'daily life' rituals and what houses and apartments might look like (over a wide range of socio-economic and geographic types). Your own thoughts and reflections come from you. Some future historians might just use data collation to plunder lots of blogs for common patterns. Others might want to do a micro-history of one person or one family's life. Who knows? It's all ways to Do History.

But in the end, honestly, not one thing I said matters in the least next to one overarching point: access. If future-me doesn't know your record exists (even knowing it exists without being able to find it, is something I can work with), that's the one time it's worthless.

Unfortunately, I can't tell you how to preserve it in a way that future historians will be able to find. Technology changes; access changes. Problems are unforeseeable. The manuscript designated Cotton MS Vitellius A XV was damaged in a fire in the 18th century after surviving 700+ years--and this is our only surviving medieval edition of Beowulf.

As a historian, I think it's awesome that you want to keep a record for history, and that you're actually doing it.

As a person, I mostly just care that you're enjoying what you do rather than seeing it as a grudging task.