The twelve months in the Muslim Calendar are as follows:

  1. Muharram
  7. Rajab
  2. Safar
  8. Sha'ban
  3. Rabi al-awwal
  9. Ramadhan
  4. Rabi al-tan)
  10. Shawwal
  5. Jumada al-ula
  11. Zul-qadda
  6. Jumadu al-taniya
  12. Zul-hijja

According to the Holy Qur'an, which expressly makes the moon the measure of time, the beginning of the month is established by observance of the new moon. "It was He (Allah) that gave the sun his brightness and the moon her light, ordaining her phases that you may learn to compute the seasons and the years."  Holy Qur'an, Ch.10 - V.5
  Because of the system of reckoning by lunar months, the Muslim year falls about 11 days behind each solar year. Thirty, three lunar years are therefore almost equivalent to thirty-two solar years. From this proportion we get the approximation formulae for transforming years A.H. into years A.D. and vice versa:A.D. = 32/33 (A.H.+622) or A.H. = 33/32 (A.D.-622)
  Instead of the old names of the days of the week prevalent among the pagan Arabs, the Arabic Muslim Calendar has the cardinal numbers in altered forms from Sunday to Thursday; Friday is the 'day of assembly'. The names of the days in the Arabic Muslim Calendar are as follows:

Yawm al-shad (Sunday)
Yawm al-Ithnain (Monday)
Yawm al-Thalatha' (Tuesday)
Yawm al-Arba'a (Wednesday)
Yawm al-Khamis (Thursday)
Yawm al-Jum'a (Friday)
Yawm al-Sabt (Saturday)

  In the days of the week, it should be remembered that Yawm al-shad begins on the evening of Saturday, Yawm al-Ithnain on the evening of Sunday and so on, so that the Arabic and European names do not cover exactly the same 24 hours.  Wheras the names of the months in the Muslim Calendar are the same throughout the Muslim World, the names of the days are different in the Muslim Calendar of the non-Arabic speaking countries.