Well, there was long speculation about Freddie Mercury's poor health and the reasons for it, with a variety of tabloid photos of Mercury showing him looking quite ill, and unverified claims in the wilder British tabloids that he had HIV/AIDS (at a point when those tabloids were hounding celebrities to try and sell papers, and when their claims needed to be taken with a mountain of salt). However, starting from tabloid claims in 1986 that Mercury had been tested for AIDS, and through to 1991, Mercury had consistently denied such things in statements and through spokesmen as rumours.

Finally, Mercury announced that he had AIDS in a statement on November 23rd, 1991, and then passed away the next day, on November 24th, 1991. You can see a news article on the topic on November 25th, 1991 in The Guardian here. I should also say that, despite it being in 2020 a clear part of Mercury's mythology that he was gay (what with the recent biopic that partially focuses on his relationship with Jim Hutton), this was not how he was perceived for much of his career - Queen were seen as creating quite gauchely machismo-filled music by many critics, and Mercury's flamboyance was sort of seen as something like a Latin-lover trope. It wasn't clear to much of the public that Freddie Mercury was somewhere on the LGBT spectrum until after his death, though it was an open secret within the music industry.

In terms of the timeline of HIV/AIDS, Mercury was probably the most prominent death from AIDS in an Anglophone country by that point, though he was not the first prominent figure to die from AIDS-related complications (e.g., Michel Foucault in 1984, the disco star Sylvester in 1987, Keith Haring in 1990). He also was not the first massively popular seen-as-straight figure to announce he had been confirmed as being infected with HIV - Magic Johnson, the LA Lakers basketball player, had announced his diagnosis at a press conference (also announcing his immediate retirement) on November 7th, 1991 (just over two weeks before Mercury passed away).

While AIDS had been in a lot of ways swept under the carpet in the 1980s, by the 1990s there was increasing public awareness about the disease - Randy Shilts' book And The Band Played On was the big, influential book that chronicled the indifference and sweeping-it-under-the-carpet of American authorities, and that book was published in 1987. That book, along with Princess Diana famously shaking the hands of men diagnosed with HIV/AIDS in a hospital, seemed to break the previous constraints that had existed around the topic. So Freddie Mercury's and Magic Johnson's announcements were, by 1991, not the beginning of a more sympathetic public reaction to AIDS, but a continuation of it.

In terms of the public reaction to Mercury's announcement, it's very much coloured by the fact that Mercury had passed away by the point that a lot of people knew he had AIDS. An obituary in The Sun has reactions from various pop stars:

Jonathan King said: “Everyone has suspected it for a long while but we all hoped the rumours weren’t true. Freddie was without doubt the most outrageous star that rock ever produced. We could badly do with more like him now, On stage and record he could be stunningly tacky but do it with class and style like no-one else.”

Status Quo’s Francis Rossi said: “I’m shattered. I think we all feared it might be coming but when it does what can you say? It’s sad.”

American rock queen Cher, whose backing group dancer Peter Tramm died from AIDS last week, said: “Oh my God. I’m devastated. It’s a huge shock.”

Bananarama star Sara Dallin fought back tears and said: “He has always been my favourite performer and I’m very, very upset.”

In terms of the reaction from HIV/AIDS campaigners, some were unimpressed by Mercury's hiding the fact he had AIDS until basically the very end. According to Rolling Stone's obituary of Mercury:

At a London AIDS conference titled Living for Tomorrow, held just four days after Mercury died, one researcher, Dr. Roger Ingham, criticized the singer’s eleventh-hour statement. “Maybe if Freddie Mercury had revealed his illness much earlier, it would have brought discussion out into the open,” he said.

A taste of a more moralistic tabloid response to Freddie Mercury's death can be seen in what I think is a compilation of writing in the British tabloid The Star available on a Queen website:

Tragic rock star Freddie Mercury’s “Russian Roulette” sex life is likely to kill scores of his lovers, a top doctor claimed yesterday. Bisexual Freddie admitted sleeping with at least 100 people – and close friends believe there were many more.

And later on,

The Star Says: Tragic waste of a genius

The showbiz mafia must NOT be allowed to turn Freddie Mercury into a saint. An angel on stage, he was a sinner in the alleyways of his private life away from the spotlight. He debauched his enormous talent by pressing his own self-destruct button.

The Freddie Mercury Tribute Concert, held in April 1992 featured a wide array of singers (from Liza Minnelli doing 'We Are The Champions' to James Hetfield of Metallica doing 'Stone Cold Crazy') performing with the remaining members of Queen, and was billed as a 'Concert for AIDS Awareness'. Televised worldwide, there's - unsubstantiated as far as I can see but not inconceivable - claims going around the internet that the concert was broadcast to a billion people worldwide.