Because some people miss the note where Tolkien states the Shire Calendar is about ten or so days in advance of ours? ;)
actually it isn't. the Shire does set aside 5 days separate from all the weeks and months, but that is only to keep the length of the months even. and they are separated out over the whole year, so the dates do coincide fairly closely most of the time. the main difficulty is that Shire-reckoning does Overlithe (their equivalent of a leapyear) as needed rather than on a fixed 4 year schedule, which tends to result on our calender's seasonal dates go out of sync with the actual seasons every 25-33 years. the way that the Shire separated out certain holidays to keep the lengths of the months even means that in their calendar the days of the week each dates fall on does not change, the way ours does. those two factors account for the variable differences in the dating, but it is not always a fixed number ahead. over the span of decades it varies between being ahead and behind.
further Tolkien himself 'translated'** all the dates given in his works to Gregorian equivalents.. for example the hobbits had no 'september' but rather "Halimath", and the date given as the 22nd of September actually meant 22nd of Halimath", which in shire reckoning always fell on a mersday. (which Tolkien 'translated' as Thursday)
of course this is all more trivia than anything, akin to trying to use the the pre-Julian roman calender's to calculate the dates of holidays within the Gregorian calendar.. it is much easier to just use the system in common use and ignore such variables. thus the 22nd of September, regardless of calendar, becomes Hobbit's day for us Tolkien fans, the birthday of the Bagginses.
**rather, he used Gregorian dates, the way he used english names for the hobbits, then put in the appendixes the 'real dates' and 'real names' in the 'original language', keeping up the illusion that he was translating some old tome rather than writing fiction, since he had fancied his work to be something to fill the missing void in anglo-saxon mythography, of which we have almost nothing in real life aside from Beowulf.. which itself is Danish in setting, and heavily mutated by later christian concepts..