The calendars of the Elves divided the year into six long periods, each corresponding to a different season or part-season. Most of these periods were fifty-four days long, but two were longer at seventy-two days each, and Rhîw was one of these (the other being Laer, or summer). Rhíw (or its Quenya equivalent, Hrívë) marked the long cold period between the last end of autumn (Firith) and the first stirrings of the new year (Echuir).
On a modern calendar, the season of Rhîw would have run from 21 November through to the following 31 January. It should be noted that the Elves themselves did not see Rhîw as marking a passage from one year to the next. The idea of a new year starting in midwinter was one that belonged to Men; to the Elves, the new year did not begin until spring (specifically, until after the season of Echuir had passed, on a date corresponding to modern 28 March).
It should be noted that there is no h sound in this word; the combination rh is used to represent an voiceless or untrilled r.