It's almost an archaic word, but when talking about Valentine's cards, it works. The part that doesn't work well is the plurality of it. One's beloved is usually one's One and Only. So, although you probably could add an s to make it plural, it just doesn't sound right, does it? Your suggestion of beloved ones is better, but a much more common phrase is simply loved ones.
Regarding this day / these days, in your example, this day is correct, as it means today. You would use these days when your sense is more "around" you - "Whereas the streets were always clean in my youth, you see garbage everywhere these days."
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