When the original date of a specified event is ambiguous, unknown, or otherwise not exact, but you know it's that time of the year to celebrate.
"It's the project's first anniversary today."
"Well today's the date that we registered it, but remember we started working on it the friday before that, and we didnt run the first test until two weeks after that."
"Well, ok, this month is the approxiversary."
"Yeah, happy approxiversary."