Aghayan, the lifetime partner of costume designer Bob Mackie, died Monday of natural causes at his home in Los Angeles, said a spokeswoman for the Costume Designers Guild.
"He was a darling man, and I loved working with him on my films," Day said Thursday in a statement to the Los Angeles Times. "He designed some beautiful gowns for me in classic, timeless styles."
A onetime stage actor, director and producer, Aghayan launched his career as a television costume designer for "Matinee Theatre" in 1957 and went on to design the costumes for two Fred Astaire TV specials and other shows.
Aghayan, who was also costume designer for Garland's 1963-64 musical variety show, was instrumental in persuading the Academy of Television Arts & Sciences to recognize the contributions of costume designers — and he and Mackie shared the first Emmy ever awarded for costume design, in 1967 for "Alice Through the Looking Glass."
Aghayan received six more Emmy nominations and shared an Emmy in 1987 with Ret Turner for "Diana Ross: Red Hot Rhythm and Blues." He won his third Emmy in 1992 for the 64th Annual Academy Awards.
After establishing himself in television, Aghayan moved into films in the mid-1960s, with credits including "Do Not Disturb," "The Glass Bottom Boat," "In Like Flint," "Caprice" and "Doctor Dolittle."
For his work in films, he received three Oscar nominations: for "Gaily, Gaily" in 1970, for "Lady Sings the Blues" in 1973 (shared with Mackie and Norma Koch) and for "Funny Lady" in 1976 (shared with Mackie).
Aghayan also worked as a costume designer for Broadway productions and received a Tony Award nomination in 1970 for "Applause."
As costume designer for the opening and closing ceremonies of the 1984 Summer Olympics, Aghayan created 50 designs multiplied, as The Times reported at the time, "into some 11,000 costumes, most of which will be unveiled at Saturday's three-hour [opening] extravaganza in the Coliseum."
Aghayan said the spirit he wanted his costumes to convey was "as American as possible and at the same time as California and as sporty as possible." For that reason, he said, the predominant color in the costumes was white.
"White is just happy," he explained. "And it's very summer. I don't think there's anything better."
In 2008, Aghayan was honored by the Costume Designers Guild Local 892 with a Career Achievement in Television Award.
"There was nobody better in a fitting room," Rachael Stanley, executive director of the guild, said Thursday. "Whenever there was a problem trying to make something work during a fitting, Ray could come in and take a look at it and say, 'Oh, the problem is....' He had a very keen eye for details."
Stanley, who worked with Aghayan on three Academy Awards presentations, described him as "brilliant" and "versatile."
"He was just a visionary, and he was right, down to the smallest detail," she said. "Everything mattered to him, that it be just right."
Aghayan was also executive producer of the 1979 TV movie "The Tenth Month" and a producer of the 1985 TV movie "Consenting Adult."
Of Armenian descent, Aghayan was born July 28, 1928, in Tehran, where his mother was a designer of haute couture.
"So, naturally, I learned about fashion and construction as I was growing up," he recalled in a 1968 interview with The Times. "But I wanted to go into the theater, to be a director or an actor."
Aghayan, who began working for his mother after impressing her by designing a line of costumes when he was 14, told Hollywood columnist Hedda Hopper in 1965 that his family sent him to Los Angeles to study architecture but that after a few weeks in L.A. he enrolled in drama school.
"I acted in and directed plays for quite a while," he said in the 1968 Times interview. "I became a costume designer by happenstance.
"I was directing a play and found we didn't have enough money to hire a designer. So I designed the costumes. I've been a designer ever since."
Aghayan, who is survived by Mackie, continued to work and recently completed several costumes for Diana Ross' concert appearances.