It's usually said that Isaac Newton was born on Christmas Day, but
there is some ambiguity in this because England was still using the
"old" Julian calendar at the time of Newton's birth, whereas the rest
of Europe had adopted the "modern" Gregorian calendar (later adopted
by England and still in use today). According to the modern calender,
Newton was born on 4 January 1643, but according to the calendar in
force at the time and place of his birth, he was born on 25 December
1642. (It's been speculated that this fact held some significance
for the mystical side of Newton's imagination, and helps to explain
his fascination for biblical interpretation, since he can hardly
have failed to notice that he was born on Christmas Day with no
worldly father - his natural father Robert, a farmer, having died
some 3 months before Isaac's birth.)
An even trickier question is whether Newton was born in the same year
Galileo died. Galileo died on 8 January 1642 (Gregorian) and Newton
was born on 25 December 1642 (Julian). But when placed on the same
calender the two events fall in different years. To make things even
more confusing, many English of that time still considered March 25
to be the first day of the calendar year, so by the old English
reading of the Italian calender, Galileo died in 1641.