What is "Semâ"

With its current perfect form, at least 700 years of vast knowledge and moral values of the civilization has made Semâ (lit. "hearing" or "listening"; Whirling Prayer Ceremony) to take place in the hearts of many people. Semâ is a one-hour summary of a centuries-old tradition, discipline, knowledge, and wisdom.
Preparation of the Meydan (the ceremonial hall, where the whirling prayer ceremony took place) for the semâ and placement of dervishes to their places are part of the ceremony which begins with laying the post (sheepskin).
At the first stage of the ceremony dervishes individually take their premade places on the Meydan which were already there before their presence resembling that Allah just knows everything before they created. 
Singing the poetic genre extolling the Prophet’s (PBUH) virtues written by Mevlana (Naat-ı-Mevlana) refers that the first created thing was the nur (light) of Prophet Muhammed (PBUH).
The first drumbeat of the qudûm (a small double drum) symbolizes the first command; "Kun (كن)" of Allah (SWT) which means "to be" or "to exist" while the following taqsim (musical solo) improvised on a nay (reed-flute) refers to the secret of the creation since breath is blown into it.
When the dervishes strike the floor with their hands (Darb-i jalâlî) it signifies that the time of arrival on the scene has come and that they will not break their promise given in the realm of the spirits and knowing that human should be evolved by following the true path.
During the Dawr-i Waladî (ritual circumambulation of three rounds ) dervishes still wear their khirqa (long black cloaks) covering all bodies and Sikka (cylindrical hats) that symbolizes the endowed wisdom of worshiping which keeps human out of harms of ephemeral world and limitless desires of the nafs (bodily self). This maqam (spiritual "station") also symbolizes that dervishes have gained with knowledge they were not aware of before. 
The Birinci Selam (first salute) in the ceremony signifies that all dervishes are now aware of the knowledge given to them by the dastûr (permission) of the shaikh (spiritual leader) at the previous stage Dawr-i Waladî and they are liable to that knowledge which can be termed as aynu'l-yaqîn (“Vision of Certainty" or, having a glimpse of Divine Reality).
The İkinci Selam (second salute) resembles the ascendance to Nafs-i Mulhama (Inspired Ego; the third of seven main levels of nafs) from Nafs-i Ammara (Dominant Ego) and Nafs-i Lawwama (Blaming Ego) as a consequence of the first salute.
The Üçüncü Selam (third salute) refers to rising with love to witness the beauty of Allah (SWT) which was inspired to the dervishes who are absorbed into the spiritual attraction. 
The Dördüncü Selam (fourth salute) refers to an ayah from Qur'an states that the hearts find peace only in the remembrance of Allah (Rad, 28). 
The ceremony ends with Qur'an recitation to tell “All that you see is present in the verses of Allah”
A Guide for watching a Religious Whirling Prayer Ceremony of Mawlavi Dervishes
Please kindly follow the rules for watching the Ceremony:

1- Be seated before the ceremony starts.  Doors of the hall will be closed right after the announced time of the program.
2- Be seated to your designated spots as indicated on the invitation given to you by the  attendants. As you take your spots, please avoid any actions that will disturb others.
3- Be aware that the ceremony that is about to start is not an artistic show. It is a religious ceremony that deserves respect. Please act accordingly.
4- Please turn off your cellphone or keep it silent.
5- Throughout the ceremony, please do not use the flashlights while taking photographs.
6- Please do not leave your seats before the ceremony completely ends ,that is, before the last dervish leaves the scene.
7- Please  keep quiet throughout the ceremony. Children can enjoy the child playroom right next to the hall if needed.