In 2012 Zayn got a “Friday” tattoo that was revealed on one of the boys’s trips to Sweden. What we notice right away is that Harry seems to be showing off Zayn’s tattoo for him in the picture above, and running his finger across it; which seems a bit off in and of itself.
But if we take another gander, we can see that the tattoo is actually written in Harry’s handwriting. We know from previous experience they are the only two boys confirmed to have tattooed each other more than once.
There are a lot of theories surrounding the “Friday” tattoo and what it could have meant to Zayn or even Zarry. I personally believe it was an inside joke between the boys, regardless of where it may have stemmed from, and because Harry was his tattoo buddy and someone he trusted to tattoo him, it was done by Harry’s hand.
One final note, Harry is always mentioning a “friend” that seems to have very similar characteristics of Zayn (who I mentioned towards the end of my video on YT) and I noticed the tattoo gun friend he talked about on James Corden in 2015 seems to fit Zayn as well.