The twelve-day period was the time after the Ottoman Empire exiled Bahá'u'lláh from Baghdad and before He began His momentous journey to Constantinople (now Istanbul). This was the time when He announced to His followers that He was the messianic figure of He whom God shall make manifest whose coming had been foretold by the Báb.
From the Writings of Bahá'u'lláh come the following passage:
Say: This is the Paradise on whose foliage the wine of utterance hath imprinted the testimony: “He that was hidden from the eyes of men is revealed, girded with sovereignty and power!” This is the Paradise, the rustling of whose leaves proclaims: “O ye that inhabit the heavens and the earth! There hath appeared what hath never previously appeared. He Who, from everlasting, had concealed His Face from the sight of creation is now come.” From the whispering breeze that wafteth amidst its branches there cometh the cry: “He Who is the sovereign Lord of all is made manifest. The Kingdom is God's,” while from its streaming waters can be heard the murmur: “All eyes are gladdened, for He Whom none hath beheld, Whose secret no one hath discovered, hath lifted the veil of glory, and uncovered the countenance of Beauty.”
Related site: Online greeting by the New York Bahá'ís