Here is my version of a 'universal' calendar of Middle-earth. In it I tried to gather all the cardinal celebrations of people of Middle-earth (which, although known under different names with different people, practically coincide in dates). According to Appendix D (LOTR) the Elvish year (not yen , which is equal to 144 years of mortals, but coranar or sunround, a solar year), usually called loa 'growth' was broken up into periods that might be regarded either as long months ( astar ) or short seasons. These seasons were six, of which Quenya names were tuile , laire , yavie , quelle , hrive , and coire , which may be translated spring, summer, autumn, fading, winter, stirring. Laire and hrive each contained 72 days, and the other ones 54 days each.
The loa began with yestare (the first or 'desired' day, yesta+[au]re ), the day immediately before tuile , and ended with mettare (the last day, [metta]+[au]re ), the day immediately after coire . Between yavie and quelle were inserted three enderi or 'middle-days' ( ende+auri, pl. from aure).
This provided a year of 365 days. Establishing the correspondence between this Elvish calendar and modern Gregorian one, I used the principal feature of the former, namely that its reference points were always determined by the celestial positions of Sun and Moon and, consequently, the length of night and day. So the beginning and the end of the Elvish year were assumed to be respectively avernal and autumnal equinox (when night and day are equally long), that is, March 25th and September 23rd.
Thus, enderi are September 23, 24, 25. The autumnal equinox in Numenor was a great holiday called Eruhantale , 'Thanksgiving to Eru', and the Durin's day of dwarves occurred at about this time. Correspondingly, the beginning of the year was celebrated as Erukyerme , 'Prayer to Eru', while later it became known as the day of the fall of Barad-dur, the New Year of the New Reconing.
The two days of solstice, June 22nd and December 22nd, were taken as middle-points of the year. Summer solstice was known as Erulaitale 'Praise of Eru' in Numenor, or Midyear's Day in the Shire. Winter solstice, however, was not celebrated by the Elves, and since no special elvish name for it has been published so far, I took the liberty of calling it Andalome (The long night) simply because this is actually the longest night of the year.