This is the 2092nd most frequent Swedish word.
dag
"Dag" means "day" in English.
Here, 'dag' (combined into 'idag', meaning 'today') indicates a specific current day in the present context.
Idag skiner solen.
Today the sun is shining.
'Dag' in this sentence refers to a single cycle of 24 hours experienced by the speaker.
Min dag har varit väldigt produktiv.
My day has been very productive.
In this usage, 'dag' signifies a nonspecific day in the future.