Title: J̌
Content:  J̌ (minuscule: ǰ) is a letter of the Latin alphabet, derived from J with the addition of a caron (háček). It is used in some phonetic transcription schemes, e.g. ISO 9, to represent the sound. It is also used in the Latin scripts or in the romanization of various Iranian and Pamir languages (Avestan, Pashto, Yaghnobi, and others), Armenian, Georgian, Berber/Tuareg, and Classical Mongolian. The letter was invented by Lepsius in his Standard Alphabet on the model of š and ž to avoid the confusion caused by the ambiguous pronunciation of the letter j in European languages.
Title: Latin script
Content:  V for the consonant. In the case of I, a word-final swash form, j, came to be used for the consonant, with the un-swashed form restricted to vowel use. Such conventions were erratic for centuries. J was introduced into English for the consonant in the 17th century (it had been rare as a vowel), but it was not universally considered a distinct letter in the alphabetic order until the 19th century. By the 1960s, it became apparent to the computer and telecommunications industries in the First World that a non-proprietary method of encoding characters was needed. The International Organization for Standardization (ISO) encapsulated the Latin 
Title: Je (Cyrillic)
Content:  The Cyrillic letter ј was introduced in the 1818 Serbian dictionary of Vuk Stefanović Karadžić, on the basis of the Latin letter j. Karadžić had previously used ї instead for the same sound, a usage he took from Dositej Obradović.
Title: History of the Latin script
Content:  I, a word-final swash form, j, came to be used for the consonant, with the un-swashed form restricted to vowel use. Such conventions were erratic for centuries. J was introduced into English for the consonant in the 17th century (it had been rare as a vowel), but it was not universally considered a distinct letter in the alphabetic order until the 19th century. The names of the letters were largely unchanged, with the exception of H. As the sound disappeared from the Romance languages, the original Latin name hā became difficult to distinguish from A. Emphatic forms such as  and  were used, developing eventually into acca, the direct ancestor of English aitch. 
Title: J
Content:   The letter J used to be used as the swash letter I, used for the letter I at the end of Roman numerals when following another I, as in XXIIJ or xxiij instead of XXIII or xxiii for the Roman numeral representing 23. A distinctive usage emerged in Middle High German. Gian Giorgio Trissino (1478–1550) was the first to explicitly distinguish I and J as representing separate sounds, in his Ɛpistola del Trissino de le lettere nuωvamente aggiunte ne la lingua italiana ("Trissino's epistle about the letters recently added in the Italian language") of 1524. Originally, 'I' and 'J' were different shapes for the same letter, both equally representing, , and ; however, Romance languages developed new sounds (from former and ) that came to be represented as 'I' and 'J'; therefore, English J, acquired from the French J, has a sound value quite different from  (which represents the initial sound in the English language word "yet").
Title: J
Content:  J, or j, is the tenth letter in the modern English alphabet and the ISO basic Latin alphabet. Its usual name in English is jay (pronounced ), with a now-uncommon variant jy. When used in the International Phonetic Alphabet for the y sound, it may be called yod or jod (pronounced or ).
Title: History of the alphabet
Content:  u. Because the u at the time looked like a v, the double u looked like two v's, W was placed in the alphabet after V. U developed when people began to use the rounded U when they meant the vowel u and the pointed V when the meant the consonant V. J began as a variation of I, in which a long tail was added to the final I when there were several in a row. People began to use the J for the consonant and the I for the vowel by the fifteenth century, and it was fully accepted in the mid-seventeenth century. 
Title: Modern English
Content: The letter thorn (þ), which began to be replaced by th as early as Middle English, finally fell into disuse. In Early Modern English printing thorn was represented with the Latin y, which appeared similar to thorn in blackletter typeface (𝖞) . The last vestige of the letter was in ligatures of thorn, ye (thee), yt (that), yu (thou), which were still seen occasionally in the King James Bible of 1611 and in Shakespeare's Folios. ; The letters i and j, previously written as a single letter, began to be distinguished; likewise for u and v. This was a common development of the Latin alphabet during this period. Changes in alphabet and spelling were heavily influenced by the advent of printing and continental printing practices. Consequently, Modern English came to use a purely Latin alphabet of 26 letters.
Title: J
Content: 𐤉 : Semitic letter Yodh, from which the following symbols originally derive ; I i : Latin letter I, from which J derives ; ȷ : Dotless j ; ᶡ : Modifier letter small dotless j with stroke ; ᶨ : Modifier letter small j with crossed-tail ; IPA-specific symbols related to J: ; Uralic Phonetic Alphabet-specific symbols related to J: ; J with diacritics: Ĵ|Ĵ ĵ ǰ Ɉ ɉ J̃ j̇̃ J with diacritics: Ĵ|Ĵ ĵ ǰ Ɉ ɉ J̃ j̇̃ J with diacritics: Ĵ|Ĵ ĵ ǰ Ɉ ɉ J̃ j̇̃ J with diacritics: Ĵ|Ĵ ĵ ǰ Ɉ ɉ J̃ j̇̃ 
Title: J with stroke
Content:  J with stroke (majuscule Ɉ, minuscule ɉ) is a letter of the Latin alphabet, derived from J with the addition of a bar through the letter. It is used in Arhuaco to represent a sound similar to the j in English just. Its lowercase dotless form ɟ is used to represent a voiced palatal plosive in the International Phonetic Alphabet.

Question: when was the letter j introduced to the alphabet?
Answer: in Middle High German.
1524.