Hi Katie, that’s an interesting one; I think it’s more because COVID has become used as a specific name in itself, so we treat it like a proper noun rather than as a variant/instance, so it’s not one of many, but a named illness. So while we say I got ‘a cold’ or ‘the flu’, but with named illnesses/diseases/conditioned we tend to refer to them without articles (e.g. polio, monkey pox, bird flu, HIV, cancer). It could be confusing as COVID 19 is actually a type of coronavirus, but it’s had such a large cultural impact that we understand the proper noun to refer to this specific one (though this could be true of the other conditions listed too, I suppose). I hope that makes sense.

(I have updated it, in fact! 🙂
Also, strictly speaking, most style guides would spell out century names rather than use ordinal numbers, e.g. nineteenth century. And would hyphenate for adjective forms, e.g. nineteenth-century writers. I haven’t done that here for the same reason I preferred using capitals – it makes it clearer where the times are being referred to. However, in more strictly stylised format, such as print, it’d be more common to see the numbers spelt out.

Excellent question Urila, one that I frequently trip up on myself! Using title case for periods of time like seasons (Spring, Summer, Autumn, Winter) or centuries, millennia etc., can be a matter of style rather than something that has strict rules. This is because it’s arguable that names for periods of time are proper nouns, naming specific times, as with days of the week. With centuries this is most common when we use an ordinal number, not a spelt name. However, most style guides agree (as you say) that these words should be lowercase – and indeed I would defer to them now.

To be honest, it’s a bad habit of mine and I must update this post to reflect it, as I wouldn’t usually do it now – the reality is this article was written before I had compared different style conventions (which heavily favour the lowercase form).

Hi Sue, interesting question – to some degree we could just say that’s just the pattern with the prepositions. I’d suggest it’s because a morning (or day, as we can say ON Monday, for example) is a smaller timeframe, while a month is quite broad, so ON indicates a more specific time while IN indicates within a broader range.