We're heavily influenced by Waldorf pedagogy, so we'd planned to do nothing with letters until 6, but DS came into this world with other ideas! He's showing very strong signs of being gifted, precocious, very smart, whatever you want to call it.
We had NO alphabet toys and he does no TV, video, computer, leapfrog, etc. His only exposure to written letters was from story books (not alphabet books) and road signs and such.
By his third birthday he was asking tons of questions about letters. What's that one called? What word is that? Then he'd point out that "cat" and "car" sound the same and would say "cuh-cuh-cat...cuh-cuh-car". This with us not only not teaching, but kind of holding back even.
So after much deliberation I bought him a very simple wood alphabet puzzle. No introduction, no lessons--just set it on his play table. He went nuts with questions!
Letter of the day or week--HA! I wish. I tried so hard to make it interesting to just focus on one--finding things around the room that started with that letter/sound--him drawing it in rice, me drawing pictures with the letter shape representing an image. Yeah, after a few minutes it was "what's this one? and then "what's this one?"
He had all the upper case and their sounds in 2 weeks. Lower case took a day or two--because so many look like their upper-case counterpart. He's now moved on to other pursuits (non-academic)--but remembers it all.
My point is simply this--let your DC lead this one! Get him/her a letter puzzle or blocks (not anything that "teaches"--talks, etc) and if s/he just plays with it without asking about the letters/sounds--s/he's probably not ready.