The chances that all of those 40 people are born on different days are:

364   363   362   ...   326
--- x --- x --- x --- x --- x 100 = 10.88 %
365   365   365   365   365

Thus, the chances that two of them are born the same day are higher than 89%.

If one considers 57 people, chances are less than 1% that they are born on different days, hence, chances that two of them are born on the same day are higher than 99%.

NOTE: We are not taking the year of birth into consideration. People born on july 4th 1980 and july 4th 2000 are considered to have the same birthday.