Sunday is called as youmul ahad, and monday is youmul ithnain... The rest of the days in a week except Friday are called with their number... But what is the reason only Friday is called jumua?.
The linguistic meaning of "jumuah" is from the root word "jama3ah" (jeem, ayn, meem) which means to gather together. Yawm al-Jumu'ah is thus dubbed because the Muslims gather together for salaat al-jumu'ah (Friday prayers) in congregation -- this is not something you can do individually, but must be done in a group.
Speaking of day names, Saturday is "Yawm As-Sabt" and also does not follow the numbering convention.