The sun gives off U.V (ultraviolet) radiation, and it's this uv which causes us to get a tan in the summer months when the uv is stronger. People all over the world flock to the open spaces when it gets warm and love to lie about enjoying the weather, but for a new tattoo, U.V is one of the biggest problems. Have you ever noticed how anything you leave in the sun for a while starts to fade? Well the same's true for a tattoo, and if it's a new tattoo, this effect is magnified greatly. After getting you new tattoo, your skin has been damaged, and it'll take up to three months for a new layer of protective skin to cover the area fully.
Tattoo in the sun (source)

After the first 2 weeks this process has already started, and it's often suggested that you wait at least this long before your tattoo sees prolonged sun exposure, and even then, it's recommended that you wear at least spf 30 sun tan lotion on the tattoo itself.

This may sound a bit extreme, but the sun can really fade a new tattoo, and until 2 weeks have passed, putting any sun tan lotion on the affected area can just irritate the skin further, as you'll be putting chemicals on a fresh wound. So, wait a couple of weeks, apply all the aftercare tips, then, try and limit prolonged sun exposure until the tattoo has fully healed (up to 3 months), and whenever you are out in the sun for a while, wear sun protection at around 30 spf on your tattoo. This will keep the image sharp and vibrant. There's no need to live in fear of the sun, it won't be the end of the world if your tattoo fades a little, and you can always get it touched up by your tattoo artist, but proper care and precautions are the best way to protect your new art, and are strongly recommended for new tattoos. For the sun and summer worshippers out there - try and time getting your new tattoo so that it has a few months to heal before the summer arrives, so that you don't have to worry about sun and water exposure too much.