Mays hit his 500th in 1965 while Robinson hit his in 1971. Back in the day when hitting 500 home runs was a commodity for a select few both Mays and Robinson were tops in their class.
September 13th certainly does mark a special day in baseball lore.
Willie Mays Topps 1970, # 600: the eyes awaiting a home run pitch (I know it is a photo from spring training but I still love those intense "eyes")
Frank Robinson Topps 1967 #100: another shot of the "home run" eyes Ranking 4th (Mays with 660) and 7th (Robinson with 586) in career round trippers, I salute Mays and Robinson for reaching that 500 HR club on the same day, albeit six years apart.
His glove work was his main skill yet in 1969 Petrocelli found that offensive punch as well. He had career highs in HR ad BA; hitting 40 (a record at the time for AL shortstops) and batted .297. He was also elected to his second all-star team (first was in 1967). While injuries would cause Petrocelli to retire after the 1976 campaign, he did have a solid career for thirteen years; hitting 210 HR's, 773 RBI's, a .251 BA and a .970 FP.
I like this 1970 card for a few reasons (aside from the fact that I am 92 % complete of the set). Take a look at Petrocelli's face - did he just wake up or something ?.jpg)

That seemed to be the kick start Bahnsen needed as his 1968 campaign earned him that gold trophy as a Topps All-Star Rookie and more importantly AL Rookie of the Year honors in 1968. He went 17-12 in '68, throwing 10 complete games, had a career high of 162 strikeouts and the second most innings pitched of his career with 267 1/3.
(a card I do not own but it is on my "will find soon" list; thanks to google images for this pic)