“I’m celebrating six months as shift supervisor.”
Consider that “celebrate” and “celestial” come from the same root word. When someone describes stars in the night sky as celestial, she’s personifying the flaming balls of gas far away in space as familiar, bright characters in a story about someone or something else. For example, there are four stars visible in the Southern Hemisphere that, when personified or otherwise called celestial, become Crux (the Southern Cross), and is said to grant seafarers courage and inspiration.
The point is that to celebrate something is supposed to invoke the stories collected around that thing. It’s about sharing these stories with others, as storytelling is an ancient ritual as old as human society. There are certainly still plenty of opportunities to celebrate a cause or an event or a person, but usually people erroneously apply celebrating to an anniversary by default.
I will try to explain the distinction a bit more by the following two examples:
Correct: “Tonight we are celebrating the grand opening of the African American annex of the City Museum, and here are our special guests to tell us about its history and the road that brings us here today.”
Incorrect: “I’m celebrating three weeks of being vegetarian.”
The former is a gathering of invested people coming together to share the stories of something rich in story and purpose. The latter is simply an anniversary. I don’t mean to diminish the anniversary, but literally there is nothing there to celebrate.
Weddings are generally a great example of a celebration, as they often provide a venue to share the stories of how the coupling came to be. Car dealerships creating sales events on President’s Day is pretty much the opposite application of the word “celebrate”, despite its usage in their insipid commercials.
“Observe” is a far more appropriate word in many applications where the word “celebrate” is wrongly used. It would be perfectly reasonable to say “I’m observing three weeks of being vegetarian.” In this case, the speaker is simply indicating she noticed that a noteworthy period of time has elapsed since her initiative has taken effect. This is more closely in line with what an anniversary is all about.