Basic answer, now! While the weather is cool!
In the cooler months (eg winter) your skin has less exposure to the elements. The less exposure, the better it is for your tattoo !
Sweat can aggravate a new tattoo and so can the sun & salt water. In summer you’re more likely to wear lesser clothing, want to be outside in the sun, go to the beach and there’s more music festivals to be attended ! During winter your skin isn’t as exposed to these elements as much because more than likely you’re going to be more covered up and stay inside; your tattoo can heal much quicker and with a lesser chance of infection.
During the healing process, your tattoo will go through the “ugly” stages; scabbing & peeling. If you get your tattoo in the cooler months, it will be covered up and by the time summer comes, your tattoo will be completely healed and ready to be shown off!!!! 🙂
Waiting time is another reason. Most people want to get a tattoo in summer because the idea is there and they want to show it off, so winter time is more than likely going to be less busy !