He specialised in printing and distributing Protestant literature and pamphlets, and produced many small-format religious books, such as ABCs, sermons, and translations of psalms.

At this time, restrictions on publishers were relaxed, and a wave of propaganda on behalf of the English Reformation was encouraged by the government of the Lord Protector, Edward Seymour, 1st Duke of Somerset.

[3] Under Queen Elizabeth I, Day returned to his premises at Aldersgate in London, where he enjoyed the patronage of high-ranking officials and nobles, including William Cecil, Robert Dudley, and Matthew Parker.

[8] Day and Seres specialised in religious works, such as those by Robert Crowley, which were largely related to theological controversies of the time.

[7] One of those publications, a satirical poem by Luke Shepherd titled Iohn Bon and Mast Person, almost landed Day in jail.

[10] Day and Seres also translated important works of Continental Protestantism for the English market, notably Herman von Wied's A Simple and Religious Consultation in 1547.

Day found Aldersgate's foreigner-friendly attributes helpful in attracting skilled Dutch workers, whom he relied on throughout his career.

[11] Day reaped more benefits from the deal than Wolfe: the English printings were used far more extensively than the Latin ones, and the ABC was eventually appended with Ponet's Catechism.

[7] With a reputation for Protestant godliness and connections to people like John Dudley, William Cecil, and Catherine Willoughby, a successful career seemed assured for Day.

However, typographical and other evidence has convinced scholars that Day set up a clandestine press in premises connected to William Cecil in Lincolnshire,[14] and that he continued to print Protestant polemical works under the pseudonym Michael Wood.

[11][15] The "Michael Wood" pamphlets included Protestant writings by Lady Jane Grey, John Hooper, and Stephen Gardiner, and attacks on Mary and her advisors.

[19] He reunited with Seres (also recently released from prison) to produce works of Catholicism for Catholic printer John Wayland, a far cry from the Protestant polemics he printed prior to imprisonment.

[21] He produced the first edition in 1559 using a new italic font of the highest quality (probably cut by François Guyot) and a large number of impressive woodcuts.

In 1559, he obtained a patent for The Whole Booke of Psalmes, Collected into English Meter, a metrical psalter, compiled mostly by Thomas Sternhold and John Hopkins, that Day first published in 1562.

[7] Day's monopolies on these perennially popular works would be the basis of great wealth over the years and a good deal of conflict between him and his fellow stationers.

[29] The resulting lavish folio filled with woodcuts was an expensive luxury item,[30] but it sold well and Day profited from his investment.

In 1567, Matthew Parker, the Archbishop of Canterbury, commissioned Day to print a collection of writings attributed to the tenth-century Aelfric of Eynsham.

It was even larger than the first—a total of 2,300 pages in two enormous folio volumes—and at one point, Day ran out of paper (which he imported) and had to paste smaller sheets together to make do.

[38] The edition has been recorded as costing sixteen shillings, roughly equivalent to two months' wages for a skilled London clothworker at the time.