Fiorentina captain Astori was found dead in his hotel room in the northeastern city of Udine on March 4, 2018 ahead of a Serie A match against Udinese.
"Always with us, happy birthday," read a banner across the gates of the Stadio Artemio Franchi in Florence, with the club posting a picture of Astori created as a mosaic on Instagram alongside "DA13 always with us".
"It would have been better to spend it with him but he is always with us even if he is not physically here," said Fiorentina midfielder Marco Benassi.
Former clubs Cagliari and AC Milan also paid tribute to the player from Bergamo.
The Sardinian club posted a photo on Facebook with two children wearing Cagliari and Fiorentina shirts both with the number 13 and the name Astori with the message "To you, dear David".
AC Milan wrote on Twitter: "Davide Astori would have turned 32 today. Forever loved, forever missed and never forgotten by his #ACMIlan family."
Juventus and Italy winger Federico Bernardeschi, who played with Astori at Fiorentina, also took to Twitter, writing "Happy birthday my friend."
An initial post mortem suggested the 31-year-old died from bradyarrhythmia — when the heart slows so much it stops — but the causes are unclear.
Two doctors who issued medical certificates declaring Astori fit to play despite evidence he suffered from a heart condition prior to his death are under investigation for negligent homicide.
The doctors, who signed the fitness certificates which gave Astori the all-clear to play, work for public bodies in Florence and Cagliari, where the defender played for eight years.
Former Roma and AC Milan player Astori, the father of a two-year old girl, also played 14 times for Italy over a six-year international career.
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