Today, things aren’t always as clear online. Spotting the word “tonight” or “yesterday” in an article you’ve arrived at via search, you likely have to scroll back to the top of the page or peek at the URL to figure out how old the story is. By the time you figure out that “tomorrow” means “yesterday,” because this story is two days old, you’ve probably lost track of what you were reading and you’re ready to click away.
The best practice is to clearly identify the day of the week. Here’s how it’s put in the B/R Stylebook:
Days of the week (e.g. Monday) should be clearly identified in all contexts. Avoid potentially confusing references like “tonight” and “tomorrow” that would force a reader to check the submission date atop the article. Note that words like “today” and “tomorrow” may be used in quotations or in expressions that do not refer to a specific date.
That last phrase refers to usage of “today” in the sense I used it a few paragraphs ago, meaning “these days.
I'm not sure if "usage" means "the act of using the word" or "the way the word is used in a language".