It is supposed to be day one. I am sure that my next info will have people say no but, to me a little late is better than no vaccine. I have been told so many things and by different people from vet, hatchery people, friends, poultry business people some say only at day one which given them the best chance to you can give it to them later.
Day one is the best, hands down, from all the research out there. That said, there seems to be no harm in doing a second vaccination later on as long as the birds are otherwise healthy and have good vigor.
Doing a vaccination "better late than never" won't hurt (as long as the bird is healthy), but it also won't help IF the bird has already been exposed to Marek's. Since Marek's takes a while to show up, it's sometimes impossible to know if exposure has occurred or not, unless you are very certain your flock is Marek's free.
Because there are variables, people often give different answers to this. The takeaway is this: Try to vaccinate properly at hatch (before they are 36 hours old) for best results, then it's also ideal to keep them completely quarantined until they are at least 3 weeks old. Some people think longer is better, which it might be... it's hard to know for sure.