is/was

It was a day he would never forget .(past tense)

Re: is/was
Depends how traumatic it was! If it was the worst day of his life so far, then you could say this...

It is the day he will never forget. [It remains (now, present) the worst day of his life.]

But if it was just a routinely bad day, then maybe you would say this:

It was a day he will never forget. [It was (then, past) a bad day.]

Not if you are talking about somebody who is alive in the present.

My father's car broke down last Monday. Then he broke his leg. Finally he got busted for pot. It was a day he will never forget.

On July 26, 1994, my sister was promoted to Sergeant Of Marines. Later, she gave birth to triplets. In the afternoon she won $5,000,000 at poker. In the evening she was told she had won the Nobel Peace Prize. It is the day she will never forget.

On 1st July, 1916, my grandfather took part in the battle of the Somme. In the morning he saw 450 members of his regiment killed before his eyes, and in the afternoon he was struck in the leg by a German bullet. He lived until 1962, but it was a day he would never forget.