Yes it's Black Saturday, and my yellow P1 Tamagotchi Kuchipatchi decided it was time to move on to the next life.  At age 11, it is pretty old for a Kuchipatchi, since back then, this is one of the unhealthy characters.  It's the best unhealthy character you can get though.

For those who've never experienced this 1997 Tamagotchi, there is no breeding function in the pet so there are no actual generations found in this toy.  It's either they move on to the afterlife because of old age, or die due to terrible care.

So your Tamagotchi turning into an angel is actually a good thing.  It means that you've taken care of your pet to its fullest.  Seeing a ghost with a grave on the other hand means that your tamagotchi died before its time signifying that you didn't take care of it properly.

When its near death, this is what happens:
1. Your tamagotchi gets sick (a skull icon appears). Several short beeps are heard.  If you try to press any buttons, it wouldn't respond.
2. The beeps get a bit longer and longer from each interval, just like the sound of a heart monitor in a hospital.  It will eventually end in a long beep.
3. Your tamagotchi lays/leaves an egg.  This will be your next pet.
4. Your tamagochi then turns into an angel with sparkly things beside it.  If you press the rightmost button, you will see the number of years he has lived in your care (as seen in the photo above).

Yes, he leaves behind an egg.  If you press the left and right buttons together, the egg will now be in your care and will soon hatch into another babytchi for you to take care of.