There are three types of this route:
(A). «X27»-service, which is following along the route with all said stops. It serves everyday.
(B). The first type of «X27» short-service. From Brooklyn on weekdays morning buses are following only to "East 23rd Street/Fifth Avenue", and toward Brooklyn on weekdays evening buses are following from this stop. Loop to East 57th Street are not serving by this buses.
(C). The second type of «X27» short-service. On weekdays morning from Brooklyn buses are following only to "Church Street/Thomas Street" stop; and begin evening weekdays Bay Ridge-bound service from "Broadway/Thomas Street".
PS: Also exists (D)-service, which was described on a separate page «X37»
On weekdays peak hours in the peak direction runs only (B) and (C) service; loop across Madison Avenue → East 57th Street → Fifth Avenue at this time served by «X37».
Full information you can see here