Perhaps not expected but some children might want to show their love and appreciation before the 50th.
We threw a 40th Anniversary party for my in-laws. They certainly did not expect it, but I believe they appreciated both the love we demonstrated and seeing many of their old friends again.
We thought their many years of marriage were worth celebrating and (although this was not stressed) my MIL had just been diagnosed with Progressive Bulbar Palsy (PBP). If we had waited for their 45th, she would no longer have been able to communicate with speech and would not physically have been up to a party. She passed away well before their 50th Anniversary. I'm glad we did not wait.