Why did Dumbledore say there was "precious little to celebrate" specifically over the 11 years before Voldemort failed to kill baby Harry?

Question

In the first chapter of Harry Potter and the Sorcerer's Stone, Albus Dumbledore says:

"You can't blame them," said Dumbledore gently. "We've had precious little to celebrate for eleven years."

I've read these books at least seven times and somehow missed this. Voldemort was around a lot longer than 11 years prior to Harry being a little baby (infant? toddler?). So why were the previous 11 years so special/bad? Hadn't he been doing awful things for 50+ years prior?

Harry was born in July of 1980 and Sybil Trelawney's prophecy happened in 1979, so not 11 years prior to when Albus, Minerva, and Hagrid dropped off Harry at the Dursley's house in ~October 1981.

So why were the 70s so bad for the magical world?

Answer 1

Dumbledore is referring to the First Wizarding War which began in 1970.

You are right, Voldemort was around for a lot longer than the preceding 11 years, but they occurred during a war. It was during the time Voldemort was at his most powerful, when he was feared by pretty much anyone. It would be very difficult to celebrate anything while so much death and destruction is happening.

Sirius says it best in The Goblet Of Fire

"Imagine that Voldemort's powerful now. You don't know who his supporters are, you don't know who’s working for him and who isn't; you know he can control people so that they do terrible things without being able stop themselves. You're scared for yourself, and your family, and your friends. Every week, news comes of more deaths, more disappearances, more torturing … The Ministry of Magic's in disarray, they don't know what to do, they're trying to keep everything hidden from the Muggles, but meanwhile, Muggles are dying too. Terror everywhere … panic … confusion … that's how it used to be."

_{Goblet of Fire: Chapter 27 - Padfoot Returns}

Answer 2

The existing answers have the right idea, but they didn't provide the evidence that it was specifically the final 11 years that were bad.

In Chapter Four of Philosopher's Stone, Hagrid says:

Anyway, this — this wizard, about twenty years ago now, started lookin’ fer followers.

As this occurred on Harry's eleventh birthday, it would mean that Voldemort started looking for followers around ten years before Harry's first birthday, which, even without taking into account approximations, is only nine months shy of the number Dumbledore gives.

In Chapter Two of Goblet of Fire it is even more explicit:

Harry had been a year old the night that Voldemort — the most powerful Dark wizard for a century, a wizard who had been gaining power steadily for eleven years — arrived at his house and killed his father and mother.

This clearly states that Voldemort's steady gain of power took place during the eleven years preceding his downfall.

Answer 3

While Voldemort was a threat to the Wizarding World prior to 1970, that was the year he proclaimed himself as the Dark Lord and started the First Wizarding War against the British Ministry of Magic. It lasted until he attempted to kill Harry in 1981.