New calendarists

The new calendarists are people who have adopted a new calendar, thus replacing the previous one, and also people who are advocating such a change within some group. Historically, most changes of calendar have been marked by debates between those who wanted to adopt some new calendar (often seen as innovators) and those who opposed such change (often seen as traditionalists). 
In the history of Christianity, such debates were initiated after 1582, when transition from Julian calendar to new Gregorian calendar was introduced within the Roman Catholic Church. During the Early Modern period, that change has sparked debates throughout the Western Christianity, particularly in protestant countries, where proponents of new calendar were often seen as "papists". 
Eventually, new calendarists won the debate, and by the 18th century, new calendar was officially accepted in protestant countries, but even after that some minor protestant groups continued to oppose the change. In the Kingdom of Great Britain, new calendar was officially introduced in 1752.
In the same time, debates between new calendarists and their traditionalist opponents were also going on within several Eastern Catholic Churches. Those debates were focused mainly on ritual questions, and ended in various compromise solutions. The need for preservation of ritual differences, including various questions related to liturgical calendar, was consequently acknowledged by the Rome. 
New calendarists in Eastern Christianity.
In Eastern Christianity, new calendarists were often faced with additional challenges. In various Eastern Orthodox and Oriental Orthodox churches, as well as in East-Syriac Orthodox churches, activities of new calendarists have been marked by prolonged debates and recurring disputes, that in some cases resulted in internal conflicts and open schisms.
Since 1923, when the Revised Julian calendar was devised, several Eastern Orthodox Churches have introduced partial changes into their liturgical calendars. Those changes were based on the application of the Revised Julian calendar for the liturgical celebration of immovable feasts (including Christmas), thus reducing the use of the old Julian calendar to liturgical celebration of moveable feasts (feasts of the Easter cycle).
Thus, the practice of dual calendar use was introduced, and consequently accepted by senior patriarchates of Constantinople, Alexandria and Antioch, as well as: Bulgarian Orthodox Church, Church of Greece, Cypriot Orthodox Church, Romanian Orthodox Church, Czech and Slovak Orthodox Church, Albanian Orthodox Church, Estonian Apostolic Orthodox Church, and most of the Orthodox Church in America. 
Churches that still use only the Julian calendar are: Patriarchate of Jerusalem, Russian Orthodox Church, Serbian Orthodox Church, Georgian Orthodox Church, Orthodox Church of Ukraine, Ukrainian Orthodox Church (Moscow Patriarchate), and Autonomous Monastic State of the Holy Mountain.
In Eastern Orthodoxy, issues related to calendar reform did not produce break of communion or schisms between canonical churches, bit they did caused disputes and internal schisms within some churches. The result of those conflicts was the emergence of the Old Calendarist movement, and consequent creation of separate churches, thus breaking the communion with those mother churches that accepted the calendar reform.
Issues related to calendar reform were one of the main reasons for schism within the Assyrian Church of the East, that introduced new calendar in 1964. Opponents of the reform seceded and formed the Ancient Church of the East.