Laudator Temporis Acti

No one knows for certain the origin of the word almanac. According to the Oxford English Dictionary (OED), it

Appears in med.L. as almanac(h in end of 13th c., and soon after (though it may have been earlier) in most of the Rom. langs., It. almanacco, Sp. almanaque, Fr. almanach, the immediate source of which was app. a Spanish Arabic al-manākh; Pedro de Alcala, in his Arabic-Castilian Vocabulista (1505), has 'manākh, almanaque, calendario'; also 'manaḥ (probably meant for same word), relox del sol' [sundial]. But the word occurs nowhere else as Arabic, has no etymon in the language, and its origin is uncertain. See note at end of this article.

At the end of the article, the "Note. As to the origin and history of the word almanac" states:

1. The earliest notices are: 1267 Roger Bacon Op. maj. xv. (1733) 120 'Antiqui astronomi ponunt principium anni circiter principium Octobris, sicut patet in expositione tabularum, quae Almanac vocantur'; Op. Tert. xi. (1859) 36 'Hae tabulae vocantur Almanach vel Tallignum, in quibus..homo posset inspicere omnia ea quae in caelo sunt omni die, sicut nos in calendario inspicimus omnia festa sanctorum'; c 1345 Giovanni Villani Cronica XI. xli, 'Secondo l'almanacco di Profazio Giudeo, e delle tavole Toletane dovea essere la detta congiunzione di Saturno e di Giove a di 20 del detto mese di Marzo' [where the 'Tables of Toledo' (constructed c 1080 by Arzachel) again point to the Arabs in Spain]. Explanations have been offered of manākh from Semitic sources, as Arab. manay to define, determine, manā measure, time, fate; Heb. manāh to allot, assign, count; Arab. manaḥa to present, minḥat a gift, all of which fail in form or sense or both.

2. Eusebius, De Praep. Evangel. iii. 4, quotes Porphyrius as to the Egyptian belief in astrology, in horoscopes, and so-called lords of the ascendant, 'whose names are given in the almenichiaká (ἐν τοῖς ἀλμενιχιακοῖς), with their various powers to cure diseases, their risings and settings, and their presages of things future.' Notwithstanding the suggestive sound and use of this word (of which however the real form is very uncertain), the difficulties of connecting it historically either with the Spanish Arabic manākh, or with med.L. almanach without Arabic intermediation, seem insurmountable. Nor does the sense really point to such tables as those described by Roger Bacon, Chaucer, and Regiomontanus.

3. Manākh has been identified with a L. manacus or manachus, applied in Vitruv. ix. 8 (Dialling) to a circle in a sun-dial showing the months or signs of the zodiac, an origin which would well explain Pedro's word in both senses; but the true reading of Vitruvius's word is now generally agreed to be menēnaeus (Gr. μηναῖος monthly); and it has not yet been shown that the reading manacus was ever so generally known or accepted, as to make its adoption probable at the hands of any Arab astronomer in Spain. Nor has it been shown to be impossible. Of many other conjectures none are worthy of notice.

See also Walter W. Skeat, A Student's Pastime: Being a Select Series of Articles Reprinted from 'Notes and Queries' (Oxford: Clarendon Press, 1896), p. 154:

I have been referred to the curious word manacus, given both by Scheller and Forcellini, as being just possibly allied to almanac. On investigation there turns out to be no such word in the Latin language; it is a pure fiction, due to a misreading. The only reference is to Vitruvius, l. 9, c. 3 (for which read c. 8). The best edition of Vitruvius, by Rose and Müller-Strubing, Leipzig, 1867, gives menaeus, with the variants maneus, manaeus. Menaeus is merely the Greek μηναῖος in a Latin dress, and is used substantively to signify the ecliptic. This is one more instance of the soundness of the advice to 'verify quotations.'

[I hunted up this reference (with the help of my late dear friend, Mr. Boase, of Exeter College) for Dr. Murray. He gives it in 'Note 3,' at the end of Almanac.]

The OED defines almanac as:

An annual table, or (more usually) a book of tables, containing a calendar of months and days, with astronomical data and calculations, ecclesiastical and other anniversaries, besides other useful information, and, in former days, astrological and astrometeorological forecasts.

Among my favorite books are a few "almanacs" by 20th century American nature writers with entries for every day of the year. One with almanac in the title is Donald Culross Peattie's An Almanac for Moderns, which does contain occasional anniversaries, or rather birthdays of famous naturalists. Others almanacs in my collection are Circle of the Seasons: The Journal of a Naturalist's Year and A Walk Through the Year by Edwin Way Teale, and Sundial of the Seasons by Hal Borland. Here, by way of example, is Borland's Sundial of the Seasons entry for April 28:

A garden has two purposes. One is to grow vegetables which feed the gardener's body and flowers which warm his heart. the other is to satisfy his need for intimate contact with the earth and growing things. Which of these purposes is more important can be disputed endlessly, but eventually the answer lies in the gardener. Perhaps, if one ever arrives at the truth of the matter, they are of approximately equal importance; as has been said many times, man does not live by bread alone, nor by lettuce and carrots and beets and potatoes and broccoli. Man happens to be a sentient person as well as a hungry animal.

By the end of April the real gardener must get his hands in the soil. Planting seeds is a good and profitable exercise, but what he needs, down in his very marrow, is contact with the earth and communion with the rootbed of life. Atoms and molecules are all very well in their place, but soil, humus, compost, earth teeming with the sources of growth, are vital beyond measure. You don't have to know the chemistry of soil or the physics of molecular structure to feel the surge of life beneath your fingertips. You dig, you rake, you plant and you are participating in the ultimate, mysterious miracle.

Such participation is at once humbling and exhilarating. You become a partner of sun and wind and rain. You acknowledge the great forces, the primal urge. You become somewhat master of a plot of earth, encourage it to do your bidding, and eventually you reap, if you are fortunate. But at planting time you chiefly commune with the earth and cooperate with the elements and take part in Spring itself. You make contact with life.

Aldo Leopold's Sand County Almanac and Joseph Wood Krutch's The Twelve Seasons: A Perpetual Calendar for the Country are not ephemerides, with an entry for every single day. Rather they have chapters for each month of the year.