'Post day' definitions:

Definition of 'Post day'

From: GCIDE

Post \Post\, n. [F. poste, LL. posta station, post (where horses were kept), properly, a fixed or set place, fem. fr. L. positus placed, p. p. of ponere. See Position, and cf. Post a pillar.]
1. The place at which anything is stopped, placed, or fixed; a station. Specifically: (a) A station, or one of a series of stations, established for the refreshment and accommodation of travelers on some recognized route; as, a stage or railway post. (b) A military station; the place at which a soldier or a body of troops is stationed; also, the troops at such a station. (c) The piece of ground to which a sentinel's walk is limited. [1913 Webster]
2. A messenger who goes from station; an express; especially, one who is employed by the government to carry letters and parcels regularly from one place to another; a letter carrier; a postman. [1913 Webster]
In certain places there be always fresh posts, to carry that further which is brought unto them by the other. --Abp. Abbot. [1913 Webster]
I fear my Julia would not deign my lines, Receiving them from such a worthless post. --Shak. [1913 Webster]
3. An established conveyance for letters from one place or station to another; especially, the governmental system in any country for carrying and distributing letters and parcels; the post office; the mail; hence, the carriage by which the mail is transported. [1913 Webster]
I send you the fair copy of the poem on dullness, which I should not care to hazard by the common post. --Pope. [1913 Webster]
4. Haste or speed, like that of a messenger or mail carrier. [Obs.] "In post he came." --Shak. [1913 Webster]
5. One who has charge of a station, especially of a postal station. [Obs.] [1913 Webster]
He held office of postmaster, or, as it was then called, post, for several years. --Palfrey. [1913 Webster]
6. A station, office, or position of service, trust, or emolument; as, the post of duty; the post of danger. [1913 Webster]
The post of honor is a private station. --Addison. [1913 Webster]
7. A size of printing and writing paper. See the Table under Paper. [1913 Webster]
Post and pair, an old game at cards, in which each player a hand of three cards. --B. Jonson.
Post bag, a mail bag.
Post bill, a bill of letters mailed by a postmaster.
Post chaise, or Post coach, a carriage usually with four wheels, for the conveyance of travelers who travel post.
Post day, a day on which the mall arrives or departs.
Post hackney, a hired post horse. --Sir H. Wotton.
Post horn, a horn, or trumpet, carried and blown by a carrier of the public mail, or by a coachman.
Post horse, a horse stationed, intended, or used for the post.
Post hour, hour for posting letters. --Dickens.
Post office. (a) An office under governmental superintendence, where letters, papers, and other mailable matter, are received and distributed; a place appointed for attending to all business connected with the mail. (b) The governmental system for forwarding mail matter.
Postoffice order. See Money order, under Money.
Post road, or Post route, a road or way over which the mail is carried.
Post town. (a) A town in which post horses are kept. (b) A town in which a post office is established by law.
To ride post, to ride, as a carrier of dispatches, from place to place; hence, to ride rapidly, with as little delay as possible.
To travel post, to travel, as a post does, by relays of horses, or by keeping one carriage to which fresh horses are attached at each stopping place. [1913 Webster]