What do instructors need to know to accommodate Muslim students? Increasingly, my school has been recruiting students from Central Asia, so I see 1-3 Muslim students in each section.

Near the end of the last term, one student asked for leave for some religious activity. He was surprised when I said he could go, then he told me he had missed many Friday afternoon religious activities, but his advisor (or perhaps some other school administrators) said he couldn't leave. I heard a similar story from another student.

I realized that many of the other Muslim students may have similar problems, but they are too nervous to speak out and let me know. Since they remain quiet, I'm not sure how to accommodate them.

What are some typical things a teacher can do to accommodate Muslim students?
There are a lot of places you could go to learn about Islam, starting with wikipedia.  It could be interesting to do so: I recently read the Autobiography of Malcolm X and found his description of his pilgrimage to Mecca fascinating and moving.  Last semester when I talked about Gabriel's Horn in my calculus course I wanted to be more balanced in my allusions, so I mentioned that in Islam the horn is blown not by Gabriel but by Israfil, and I was strangely pleased to figure out for myself that Israfil is the Islamic counterpart to Raphael (whereas Jibrail's role is expanded from that of his Christian counterpart).
The point of that preamble was: I do not doubt that learning more about Islam would be a worthy endeavor.  Nevertheless I am skeptical that such knowledge would directly help you to accommodate Muslim students. Like most major world religions, there is considerable variation in the way it is practiced.  I recommend rather that you familiarize yourself with the policies of your university on religious accommodations.  Just yesterday I received the yearly memo on Sensitivity to Religious Practices from my upper university administration.  It reads:

Many of our faculty, staff, and students commemorate various events of importance to their particular religions. Our institutional practice is to make every reasonable effort to allow members of the University community to observe their religious holidays without academic penalty. Absence for religious reasons does not relieve students from responsibility for any part of the course work required during the period of absence. Students who miss classes, examinations, or other assignments as a consequence of their religious observance should be provided with a fair alternative opportunity to complete such academic responsibilities. Students must provide instructors with reasonable notice of the dates of religious holidays on which they plan to be absent.
As you plan your syllabus and begin communicating with students, please keep in mind that some religious holidays affect a significant number of University of Georgia students and might require a student to abstain from secular activities or attend a house of worship. Different groups within a particular religion may also observe holidays on different dates, making it difficult to provide a comprehensive list of all potential religious observances. You may wish to search online for a religious calendar resource to serve as a guide for the dates of common observances.
Thank you for your cooperation.

At least if you are in the US (as I seem to recall is the case?) it seems very likely that your institution has some equivalent version.  You could keep copies on hand and give them to a student like the one you described above.  You can direct them to appropriate university administration if they feel that they are not being accommodated within the stated rules of the university.  If you really feel strongly, you could try to speak to the relevant administrators yourself.