This slightly crumpled piece of paper grants you access to all areas of Vault City, except the Vault itself, during daylight hours only.
Weight: 45 grams.
With the Day pass you can enter part of Vault City called Inner City without being a citizen. But only during the day (8:00-21:00). You will not be allowed to enter Vault 8 itself, that area is for citizens only.
Day passes are tied to the character who bought them. Characters trying to use a Day pass purchased by another character will get shot when trying to enter the Inner City.
Talk to Wallace in the Customers Office in the Courtyard. He will offer you a Day pass for 500 caps but you'll need to have at least 3500 caps to impress him. If your Charisma is less than 3, he will call Skeev, his assistant, to get rid of you.