By adding two Muslim holidays to the school calendar, Mayor Bill de Blasio no doubt made a lot of future voters very happy.

In a public school system where at least 1 in 10 students is Muslim, this shows respect for their faith and also reflects the growing diversity of the city's students.

Eid ul-Fitr marks the end of the month of Ramadan. Eid ul-Adha commemorates the completion of the pilgrimage to Mecca. They are Islam's holiest holidays. Recognizing them, beginning in the 2015-16 school year, means Muslim students won't have to take an excused absence to observe the holidays with their families. And the teaching plan in schools with a large number of Muslim students won't be disrupted because so many did stay home.

Fid-ul-Fitr on Sunday, July 5, after the regular school year ends will only affect the summer school schedule. Eid ul-Adha will be marked on Sept. 24 next school year. But because Islam follows a lunar calendar, those dates will change from year to year.

As the holidays come around, they will present opportunities for all students to learn about Islam. That's a start: It's a way to fight school bullying and stereotypes of Islam defined by the extreme violence in the Mideast.

De Blasio kept a campaign promise when he recognized the holidays -- the first religious ones added to the city's school calendar since Yom Kippur and Rosh Hashanah in 1960-61. But members of other faiths are sure to push for similar recognition, so decisions about which holidays to observe shouldn't be so ad hoc. City officials should set some benchmark -- for example, when adherents of a particular religion top 10 percent of the student population -- to trigger recognition of some holidays, such as the Chinese Lunar New Year or Diwali, the Hindu festival.

But in a competitive global economy, American students need more days in school, not fewer. While the Department of Education says no instructional days will be lost this time around, if it adds other holidays, the department should consider lengthening the state-mandated 180-day academic year or shortening the February winter break.