You are very pretty today
Imagine a woman is going out and she dresses up. Then, someone says: you are very pretty today.
I am not sure if "pretty" is the expression for this provisional situation.
Thank you.
4 Answers
the temporal part would be "today"...
hoy estás preciosa (Today you look pretty) eres preciosa (you are pretty)
Yes, Nila, the word "pretty" is acceptable here. However, we more often do not use "today" in the expression. It is common to say:
"You look very pretty."
"You look great."
"You look amazing."
Usually, by the tone of someone's voice, they know exactly what you mean. If you say "You look pretty today.", that might imply that the person usually looks just average or even not very attractive on most days. But today, they actually look pretty. So if you use the expressions I gave above and say it with a lot of enthusiasm, the person will take it that you are complimenting the effort put forth to look nice for a special occasion, but not that they aren't pretty on other days. 
You can also focus on the dress itself.
"The dress you have on today is very pretty."
Edit: sorry I previously answered from a Spanish language perspective, it has been answered fairly well now from the English perspective and I agree with the comments above.
We would rarely say 'today' instead the difference between how you look in this moment would be shown by the word 'look' - you look beautiful! (at this moment) rather than - you are beautiful (I always think this and am just taking this moment to tell you)
I would be inclined to say beautiful or gorgeous rather than pretty for a woman and pretty for a little girl but that may just be me. 
te ves muy linda / estás muy linda ------> you look very pretty
eres muy linda -------> you´re very pretty
te ves muy linda hoy ------> you look very pretty today (not yesterday, maybe not tomorrow)
Gather ye rosebuds while ye may,
Old Time is still a-flying:
And this same flower that smiles to-day
To-morrow will be dying.
(Robert Herrick)
