Where I'm from, we often say “with the day that's in it” when referring to a special occasion. For example, on Valentine's Day I might say to my girlfriend “Let's go out for dinner, with the day that's in it.” Or if it's the anniversary of someone's death, I might say “I went to the graveyard earlier, with the day that's in it.”

Is this phrase a borrowing from Irish? Would Irish-speakers say “leis an lá atá ann” in that context.

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

I don't know about the "with," but I've certainly heard "an lá atá ann" a lot, so I'm pretty sure that does come directly from Irish.

Redwolf

I'd say agus an lá atá ann (inniu). But I'm not a Gaeilgeoir ó dhúchas.

with the day that's in it...is a real Irish phrase in English and perhaps it had a Gaelic beginning. It sounds so. However, I know of no Gaelic equivalent in modern Gaelic/ Irish that would capture the beauty of that phrase. "Leis an lá atá ann" is the translation but that would never be said. If I were to say this in Gaelic, I would say,

" A dheara, na laethanta so...." but I think the English phrase captures a great deal here that the Gaelic Language doesn't.