There is no point of no return as long as you don't save after getting the bad/normal ending. To get the true ending, you obviously have to be on the normal path. To get the normal ending and go to the true ending path (major spoilers in here, you've been warned!):
Do not throw Namatame in the TV. Convince Yosuke to calm down. You'll then have to choose from a list who you think the true killer is. Pick Adachi. Finish the next dungeon. After that, talk to all of your friends on 3/20 and go to Junes. Choose not to go home. Do this a second time, and the Investigation Team will gather up at the food court for a final time. You'll get a letter from Adachi, telling you that he thinks there's a true mastermind behind everything. After that, go to the Velvet Room, then talk to the gas station attendant. You have to pick specific choices here, but you can't screw up; just keep trying or look it up. Follow Izanami into the TV. This is the true final dungeon.
This is for the PS2 version, but it should be a similar deal here. I think you may have to max Adachi's social link to get the true ending path in this game but I don't know for sure.
I'd say after beating the Heaven dungeon. Correct me if it's changed (those who imported), but finishing Heaven was the first point the ending is decided, but not the last. Screw up there, and you get the bad ending. Pass the 'test' and you keep going, only to face another 'test'. That is the real difference between the true ending and the... Normal (?) ending. Just deny it's the end. Convince the game it's not ove and that there are unanswered questions, and it will open the true last dungeon. --- PSN: XIII_Lightning XBL: xDagger1087
Well since the topic itself doesn't have a spoiler warning, I hope anyone that does post spoilers uses spoiler tags
Basically, save your game on December 2nd.
These are basically your ending options. There are a total of 4 endings (arguably more because I think in some of them there are really minor dialogue changes, but as far as the actual ending does, it's 4)
December 3rd - Throw Namatame into the TV world - Bad Ending
December 5th - Fail to identify the murderer in 3 guesses - Bad Ending
Also December 5th - If you've been doing the Pierrot social link, and you identify the true killer, you'll have the option of not telling your friends who it is. The game will skip ahead to March 30th, the MC will talk to the murderer and if you choose all the right dialogue options, you'll get the brand new Accomplice ending. If you screw up any of these dialogue options, you'll just be put on the Bad Ending path. Also, you make some choices when trying to hide the killer from your friends. If you mess these up, you'll reveal who it is and be on the path to the normal ending - Accomplice/Bad Ending
Once you're past December 5th if you didn't do any of that stuff, you're basically on path for the Normal ending.
As for the True Ending On the final day, March 30th, save your game then. You'll talk to all your social links and stuff, the game asks if you wanna go home. Say No. Then go to the food court, keep telling the game you do not want to go home. This will open the true ending. There are steps you take after that. Easy enough to look up on your own
ALSO, sort of an add-on to the True ending...
If you max out the Aeon social link, be sure to swing by the Velvet Room on either January 1st or January 2nd, I forget exactly which one. This will open an optional dungeon and that lead to a very special extended epilogue on the True ending
Hope that helps, lemme know if you have any more questions.
But yes, just save like right at December and you should be fine. --- Don't forget to check out my MegaTen themed webcomics at http://www.drunkduck.com/user/Strain42/
It's worth noting this is no "point of no return" exactly. Picking the wrong choice immediately brings you to the ending. There no chances to save after wards so it's not possible to pick the wrong choice then accidentally save, locking you out of the ending.