The The Advent Series is an (as of newpage 2638) ongoing series of daily posts by forum user The Advent in the One True Thread combining photography and poetry. It began on newpage 2635 and concluded on newpage 2640, with a a followup post on the same newpage where BlitzGirl revealed herself as the author.
The posts in the series have no post titles using the corresponding field provided by the forum software. However, each post in the series begins with a centered, large-sized caption containing a number (starting from 1), a colon and a keyword that is related to events and plot elements of the One True Comic, such as 1: Beach. Next comes a greyscale (black and white) photograph 553 pixels wide and 395 pixels high, i. e. exactly matching the size of the One True Comic's frames. This is followed by a spoiler containing a poem (with title and author attribution) which is in its turn somehow topical to the noun in the caption and/or to related events in the One True Comic. Finally there is the image that has been shown previously shown as a post attachment (cf. Redundant Spoilers) and in some cases a comment that in one way or another responds to current goings-on in the thread.
The final post in the series on Syzygy, 4 A.T., employed a slightly different format where instead of the usual photograph, The Advent's avatar image is placed above the caption and then after the caption there follows a spoiler containing all of the photographs of the previous posts in the series. After this spoiler there follows, as usual, a poem and then finally a second spoiler containing the First Commandment.
The first post in the series was posted at 5:00 am UTC on 24 February 2018 (heretical), i.e. 1 Advent, 4 A.T., and therefore in the run-up to the fifth timiversary. Subsequent posts were timed at one post per day, almost always at 5:00 am UTC as well, which suggests the usage of a bot.
Starting from number 17 in the series, on 12 March (17 Advent), the posts in the series were posted at 4:00 am UTC. This change coincides with the beginning of daylight saving time in most of the USA and Canada. In particular, the usual posting time has consistently been at midnight in the US/Canadian Eastern Time Zone.
The final post came on 24 March 2018 (Syzygy, 4 A.T.), again at 4:00 am UTC, and the followup post by BlitzGirl on 25 March 2018 (1 Innocence, 5 A.T., the timiversary) at the same time.
The first post in the series was - presumably by mistake - originally already posted on 23 February 2018 (28 Community, 4 A.T.) and subsequently deleted again. However, it had already been noticed by other OTTers who during the following hours reconstructed and re-posted the post, albeit with a smaller version of the original image. This was rendered mostly moot when The Advent re-posted their version of the post, including full-sized image, one day later.
The identity of the author of the posts was unknown until BlitzGirl's revealing post on the timiversary.
The user account of The Advent was only registered on 22 February 2018 (27 Community, 4 A.T.), on which day the user also posted five comments on various threads. Since then, they had only posted the subsequently deleted post on 23 February (28 Community) and after that the posts of the ongoing series, but nothing else. It has been speculated that the five posts of the first day were only made because the first five posts of each account on the xkcd forums are moderated, so that the posts of the actual series which seem to have been the actual point of creating the account would from the beginning not be subject to a moderation-related delayed appearance in the thread.
It seemed plausible that the author of the posts was an experienced OTTer who had previously appeared on the One True Thread with another username. This speculation was subsequently corroborated by BlitzGirl's reveal.
The time of the posts being generally consistent with midnight in the US/Canadian Eastern Time Zone, including the changeover to daylight saving time, in the end turned out not to be an indication of The Advent's location (BlitzGirl being based in the Pacific Time Zone). However, the Eastern Time Zone includes the location of the One True Author's residence, and it seems possible that the bot was for that reason set up to post using that particular time zone.
The poems in the posts are attributed to various authors and therefore seem not to be original content. According to a statement by The Advent on NP2636, the photographic images are also based on works by others, but modified by The Advent.