Question: What is a significant mosque in Istanbul, made of two former Eastern Orthodox churches and a chapel,  Istanbul High School, or Zeyrek Mosque?

Context articles: Revised Julian calendar
Kefeli Mosque
Fenari Isa Mosque
Balamand declaration
Intercession of the Theotokos
Old Calendarists
Koca Mustafa Pasha Mosque
Istanbul High School
Zeyrek Mosque
Anglican and Eastern Churches Association

The Revised Julian calendar, also known as the Milanković calendar, or, less formally, new calendar, is a calendar, developed and proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide.
 This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church.
 The Revised Julian calendar temporarily aligned its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world.
 The calendar has been adopted by the Orthodox churches of Constantinople, Albania, Alexandria, Antioch, Bulgaria, Cyprus, Greece, Poland, and Romania.
The Kefeli Mosque (Turkish: "Kefeli Câmîi" , meaning "the mosque of the Caffariotes", after the inhabitants of the city of Caffa in Crimea, or also "Kefeli Mescidi", where "Mescit" is the Turkish word for a small mosque) is a former Eastern Orthodox church, later jointly officiated by Roman Catholics and Armenians, and finally converted into a mosque by the Ottomans.
 The Catholic Church was dedicated to Saint Nicholas.
 Its date of dedication as Eastern Orthodox church is unknown.
 The interest of Kefeli Mosque arises because it reproposes the early Christian Basilica form during the later Byzantine period.
Fenâri Îsâ Mosque (full name in Turkish: "Molla Fenâri Îsâ Câmîi" ), in Byzantine times known as the Lips Monastery (Greek: Μονή του Λιβός ), is a mosque in Istanbul, made of two former Eastern Orthodox churches.
Uniatism, method of union of the past, and the present search for full communion, also known as the Balamand declaration and the Balamand document, is a 1993 report written by the Joint International Commission for Theological Dialogue Between the Catholic Church and the Orthodox Church during its 7th plenary session at Balamand School of Theology in Lebanon.
 The report discusses ecclesiological principles and suggests practical rules for both the Catholic Church and the Eastern Orthodox Churches to implement about improving relations by reciprocally avoiding interfering in each other's Churches and not using history in a polemical manner.
 According to Cardinal Edward Cassidy, the report contains three principles: that individuals have the freedom to follow their conscience, that Eastern Catholic Churches have the right to exist, that uniatism is not the current method of full communion; and two conclusions: that the Catholic Church and the Eastern Orthodox Churches are "sister churches" and that rebaptism should be avoided.
 The Eastern Catholics rejected the report "because it seemed to imply they should never have existed in the first place" while the Eastern Orthodox rejected it because it did not call for the abolition of the Eastern Catholic Churches.
The Intercession of the Theotokos or the Protection of Our Most Holy Lady Theotokos and Ever-Virgin Mary, known in Church Slavonic as Pokrov (Покровъ, "protection"), and in Greek as Sképē (Σκέπη), is a feast of the Mother of God celebrated in the Byzantine rite Churches—principally the Eastern Orthodox.
 The feast celebrates the protection afforded the faithful through the intercessions of the Theotokos ("lit."
 Mother of God, the Eastern version of the Virgin Mary).
 In the Slavic Orthodox Churches it is celebrated as the most important solemnity besides the Twelve Great Feasts and Pascha.
 The feast is commemorated in Eastern Orthodoxy as a whole, but by no means as fervently as it is in Russia, Belarus and Ukraine.
 It is not a part of the ritual traditions of, and therefore is not celebrated by, the Oriental Orthodox Churches or Western Rite Orthodoxy.
 Yet the feast is perfectly consistent with the theology of these sister churches.
An Old Calendarist is any Eastern Orthodox Christian or any Eastern Orthodox Church body which uses the historic Julian calendar (called "Old Style Calendar" or "Church calendar" or "Old Calendar"), and whose Church body is not in communion with the Eastern Orthodox Churches that use the New Calendar.
 The "Old Calendarists" (who are also sometimes styled "Old Calendar") are to be distinguished from Eastern Orthodox Christians or Eastern Orthodox Church bodies which are "on the" Old Calendar.
 The latter use the historic Julian calendar cited above, but "are" in communion with the Eastern Orthodox Churches that use the New Calendar (the Revised Julian calendar).
 Thus, to be "Old Calendarist" or "Old Calendar" is not the same thing as being "on the Old Calendar".
 The Russian Orthodox Church, for instance, is not Old Calendarist (or Old Calendar), but it is on the Old Calendar.
 There are a great many Eastern Orthodox Christians who are (or who belong to Churches that are) "on the Old Calendar", but far fewer in number are the Eastern Orthodox Christians who are "Old Calendar" or "Old Calendarist."
 It also should not be confused with the Oriental Orthodox churches, all of which are either "on" the Old Calendar or use their own calendar, but who are not in communion with either the Old Calendarists or mainstream Eastern Orthodoxy, although they are currently engaged in ecumenical dialogue with the latter.
Koca Mustafa Pasha Mosque (Turkish: "Koca Mustafa Paşa Camii" ; also named "Sünbül Efendi Camii") is a former Eastern Orthodox church converted into a mosque by the Ottomans, located in Istanbul, Turkey.
 The church, as the adjoining monastery, was dedicated to Saint Andrew of Crete, and was named Saint Andrew in Krisei or by-the-Judgment (Greek: , "Monē tοu Hagiοu Andreοu en tē Krisei").
 Although heavily transformed during both the Byzantine and the Ottoman eras, it is one among the few churches in Istanbul still extant, whose foundation goes back to the sixth century.
İstanbul High School, (Turkish: İstanbul Lisesi , German: Istanbuler Gymnasium ) also commonly known as İstanbul Erkek Lisesi, abbreviated İEL, is one of the oldest and internationally renowned high schools of Turkey.
 The school is considered elite among Turkish public high schools.
 Germany recognizes the school as a "Deutsche Auslandsschule" (German International school).
Zeyrek Mosque (full name in Turkish: "Molla Zeyrek Camii" ) or Monastery of the Pantocrator (in Turkish: "Pantokrator Manastırı" ), is a significant mosque in Istanbul, made of two former Eastern Orthodox churches and a chapel.
 It represents the most typical example of architecture of the Byzantine middle period in Constantinople and is, after Hagia Sophia, the second largest religious edifice built by the Byzantines still standing in Istanbul.
The Anglican and Eastern Churches Association is a religious organization founded as the Eastern Church Association in 1864 by John Mason Neale and others and of which Athelstan Riley was a leading member.
 The purpose for which it was founded is to pray and work for the reunion of the Eastern Orthodox Churches and the Anglican Communion.
 In 1914 it adopted the present name when it merged with the Anglican and Eastern Orthodox Churches Union.
 According to tradition the merger was arranged at a meeting under a railway bridge in Lewisham between the Rev. H. J. Fynes-Clinton and the Rev. Canon J. A. Douglas.
 In 1933 there was a dispute between Fynes-Clinton and Fr Robert Corbould on one side and Athelstan Riley and Douglas on the other.