I just say that it comes naturally with being God's Favorite Atheist.

a good day

Plinko.

I use to belong to a group of crazies. Yes , me too. Anyway, one of the key persons with all the answers would say she was sending light and positive energy to people. One day she was in a bind and was saying how desperate she was. The other members of the group starting telling her they were sending her light and positive energy. That truely screwed her head up. That was the last we heard from her. Lol.

coincidence

In the right place at the right time.

Luck or irony

I don't know how life would hand out anything, but I get the metaphor. Serendipity occurs when looking for one thing and 'serendipitously' finding something else, particularly in research. "He was looking for a formula to produce honey and found a cure for ___."

Anyway, applying it to the specific question, serendipity is serendipity. Any other name one puts on it shows a leaning to the unreal side of life. "God did it." "My lucky number came up." "Must have been in the stars." "The Great Pumpkin smiled on me today." "The unknown force at the center of the Universe is responsible."

I'd call it an accidental happening, unless 'serendipity' fit the situation better.