Learn English – the difference between “during a day” and “during the day”

They are very close semantically. Phonetically, they are a little different. We cannot put phonetic stress on the article a. But we can put phonetic stress on the word one. When we say "one feature of my work," there is therefore an emphasis on the one.

Most commonly, and is as exemplified by your sentence, this emphasis "emphasizes the indefiniteness of the article," i.e. it emphasizes that this is just one of many. Perhaps you don't want the reader to think that this is the only feature of your work. The "one" emphasizes that there could be more.

"Next day", without, "the" sounds very strange to me (native Australian English speaker). The only possible common usage might be in advertising copy, e.g. "We offer next day service" - but generally that kind of advertising does not reflect good grammar or even normal usage.

I would always say "the next day".

Note that in your second example, omitting "the" doesn't indicate that the boots are ready on the 4th, as opposed to the 2nd. You need to leave "the" in there, but the sentence is ambiguous anyway.