|In the Catholic faith, the whole month of May is dedicated to the Blessed Mother Mary. So it is fitting that we celebrate Mother's Day in May.|
Below is the explanation about the tradition. I found it on a site about Catholicism.
By tradition, the Catholic Church dedicates each month of the year to a certain
devotion. In May, it is the Blessed Virgin Mary. This devotion arose among Jesuits
in Rome in the late 18th century and quickly spread throughout the Western Church.
By the time of Pope Pius IX's declaration of the dogma of the
Immaculate Conception in 1854, it had become universal.
May crownings and other special May events in honor
of Mary, such as public recitation of the rosary, stem from this time.
Wiki answers has this to identify which countries celebrate Mother's Day in May:
In the United States and many other countries, Mother's Day is always celebrated on the
second Sunday in May. The other countries that celebrate Mother's Day at this time are:
Anguilla, Aruba, Australia, Austria, Bahamas, Bangladesh, Barbados, Belgium, Belize, Bermuda,
Bonaire, Brazil, Brunei, Bulgaria, Canada, Chile, China, Colombia, Croatia, Cuba, Curaçao, Cyprus,
Czech Republic, Denmark, Ecuador, Estonia, Finland, Germany, Ghana, Greece, Grenada, Honduras,
Hong Kong, Iceland, India, Italy, Jamaica, Japan, Latvia, Malta, Malaysia, Myanmar, The Netherlands,
New Zealand, Pakistan, Peru, Philippines, Puerto Rico, Singapore, Slovakia, South Africa, St. Lucia,
Saint Vincent and the Grenadines, Suriname, Switzerland, Taiwan, Trinidad and Tobago,
Turkey, Ukraine, Uruguay, Venezuela, and Zimbabwe.