As we know,there are 365  or 366 days every year.
365/7=52...1, 366/7=52...2
It means 2 days earlier each 4 years
Then the circle is 28 years,he has 4 chances
80/28=2...24
And there 4 chances also at his last 24 years
Totaly 12

sunday on May 11 1997,age 1
monday on May 11 1998,age 2
tuesday on May 11, 1999,age 3
thursday on May 11, 2000,age 4
friday on May 11, 2001,age 5
saturday on May 11, 2002,age 6
sunday on May 11, 2003,age 7
tuesday on May 11, 2004,age 8
wednesday on May 11, 2005,age 9
thursday on May 11, 2006,age 10
friday on May 11, 2007,age 11
sunday on May 11, 2008,age 12
monday on May 11, 2009,age 13
tuesday on May 11, 2010,age 14
wednesday on May 11, 2011,age 15
friday on May 11, 2012,age 16
saturday on May 11, 2013,age 17
sunday on May 11, 2014,age 18
monday on May 11, 2015,age 19
wednesday on May 11, 2016,age 20
thursday on May 11, 2017,age 21
friday on May 11, 2018,age 22
saturday on May 11, 2019,age 23
monday on May 11, 2020,age 24
tueday on May 11, 2021,age 25
wednesday on May 11, 2022,age 26
thursday on May 11, 2023,age 27
saturday on May 11, 2024,age 28
sunday on May 11, 2025,age 29.

蝌蚪文害死人耶.

Counting every year is just to check and confirm.
Fortunately,the year 2000 has also 366 days.