grammar - Meaning of 今日も一日？

仕事を終えた労働者が、ダンジョンから無事戻ってきた冒険者達が、今日も一日の締めくくりとばかりに酒盛りに耽っている。

This is the first time I have come across this. What exactly does it mean and how would I use it? I've seen it in other sentences too like:

今日も一日頑張りましょう！

Thanks!

Answer

We have two very different 「今日{きょう}も一日{いちにち}」's here.

「今日も一日の締{し}めくくりとばかりに酒盛{さかも}りに耽{ふけ}っている。」

＝

「今日も、一日の締{し}めくくりとばかりに酒盛{さかも}りに耽{ふけ}っている。」

≒

「一日の締めくくりとばかりに、今日も酒盛りに耽っている。」

Here, 「今日も ("today also")」 and 「一日 (the "work" day)」 function separately and independently from each other. Grammatically, it is not the same 「今日も一日」 that we use very often as a set phrase.

"Today, too, (they are) indulged in a drinking party as if it were the closing ceremony of the day."

Moving on...

The insanely common set phrase that I am talking about is the "other" 「今日も一日」 seen below.

「今日も一日頑張{がんば}りましょう！」

Here, 「今日も一日」 means "today as usual" or even just "as always". It works adverbially.

"Let us work hard (today) as usual!"