Britain was fairly late in switching from the Julian to the Gregorian calendar (the latter was, after all, a Popish innovation). It only did so in 1752, as a result of the Calendar (New Style) Act, often known as Chesterfield's Act, passed a couple of years before. Noting that there were inconveniences in having a calendar that was so different from its neighbors, and a need to reduce confusion in matters of dating, this act made two changes: first, the official beginning of the year would be January 1 rather than March 25 (Scotland, while still on the Julian calendar, had already made this change a century and a half before); and the Gregorian calendar, denominated the New Style, would be used from then on. Switching to the latter required a little finagling of dates given the differences between the Julian and the Gregorian calendar by that time. In September 1752, the calendar was advanced several days -- one day it was September 2, and the next it was September 14. Because of this there is perpetual confusion about dates in 18th century Britain: dates before the turning point were sometimes converted to New Style and sometimes not, a few holdouts here and there continued to use Old Style, and people often mention dates without indicating whether they were New Style or Old Style. You always have to take dates in 18th century Anglophone countries with a grain of salt.
David Hume was born David Home to Joseph Home of Chirnside and Katherine Falconer on April 26, 1711 (Old Style) -- that is, this was his birthday in the Julian calendar that was regnant when he was born. Converting that date into New Style gives us May 7, 1711 (New Style). Thus it's an open question which day to celebrate Hume's birthday; if you prefer just sticking with the Old Style date, then you would celebrate it on April 26. You may have noticed that the Humean theme around here started up at that time. But if you want to celebrate the date as it would have been in our current calendar, you celebrate it today, on May 7. And this year Hume's birthday seems to deserve a bit of a mention, since Hume would be 300 years old, and over the course of that three centuries he has certainly made his mark felt.