The al- prefix of the word "almanac" points to a borrowing from Arabic, but, as the OED says, "the word occurs nowhere else as Arabic, has no etymon in the language, and its origin is uncertain." The earliest known dating is from 1267 by Roger Bacon, but the first known use in English is from Chaucer's 1391 Treatise on the Astrolabe, and actually constitues a very recognisable definition: "A table of the verray Moeuyng of the Mone from howre to howre, every day and in every signe, after thin Almenak."
The well-established almanacs such as The Old Farmer's Almanac in America and Old Moore's Almanac (previously Vox Stellarum) in the United Kingdom and Ireland are curious for the fact that they have stuck apparently closely to their formats since their establishments in 1792 and 1700 respectively. Such has been the popularity of almanacs even in recent times that in 1943 the Dáil Éireann, a house of the Irish Parliament, debated why volumes of Old Moore's Almanac had been officially seized from stationers and defaced:
Mr. O'Donovan: Was the Minister afraid that the predictions in Old Moore's Almanac would affect the morale of our people?
Mr. Davin: Did Old Moore say when the War would end?
Mr. Cogan: Was the Minister's decision affected by the fact that Old Moore forecast the defeat of the Government at the next election?
Mr. O'Donovan: And that the stars are against him?
It's an amazement to me, then, that the Web is not as straightforwardly replete with almanacal—or almanacy, which Google seems to prefer—information as I expect it to be. There are many great sites, e.g. Heavens Above for astronomical data, BBC Weather for meterological data, and HM Nautical Almanac Office for everything else (I love their motto: Man Is Not Lost), but in these days of info-glut and the clear superiority of sites such as Google that provide unfettered presentation of data, I really would expect there to be a single site that could incorporate all of this whilst perhaps even retaining the scrap-book style of the traditional almanacs, updated for the current millennium.
Moreover, I think that the importance of customisation cannot be overestimated. Whilst I'm not a great fan of Web-services, a Web-service-like interface to these sites would enable anyone with rudimentary programming skills to set up their own interfaces—again to use Google as an analogy, it'd be much like their XML-RPC interface, enabling people to Google from IRC or the command line.
If only such information were static, it would be easy to provide, but one's location on the surface of the planet is the key variable to almanacs. At least it's something that can be handled much more easily online than in print, but even then I doubt oddnesses such as the double sunset at Leek, Staffordshire will oft' be accounted for.