The slogan Libertas quæ sera tamen is from the Minas Gerais "conspiracy" (Inconfidência Mineira). The form and design of the flag were made on the model developed by the leaders of the liberation movement of 1789 for the republic that they planned to create. According to Tiradentes [one of the leaders of the movement], the triangle recalls the three Persons of the Trinity.
The flag of the state was [officially--ed.] instituted on 8 January 1963. It consists of a white flag with a red triangle in the center, symbol of the nationalist conspiracy of 1789, known as the Inconfidência Mineira or Revolução de Tiradentes. According to the statement of Joaquim José da Silva Xavier, nicknamed "Tiradentes" (1748-1792), the most famous of the leaders of the Inconfidência, the triangle represented the three persons of the Trinity. The revolt was aborted by Portuguese colonial authorities through breach of faith (inconfidência) of one of the conspirators. Jaume Ollé, 28 June 1996, with minor corrections by Paulo Erlich, 15 December 1998
The motto in the flag means literally "Freedom although late," meaning, "Freedom is necessary although it is already late for it." The name Inconfidência Mineira is due to the fact that the conspirators were considered traitors by the Portuguese crown. The name was given by the Portuguese colonial authorities to the conspirators and has nothing to do with the traitor who denounced the conspiracy. At the time, it was used derogatorily, but later the Brazilians turned around its meaning (in this special case), making it a term of praise. What was infamous to the Portuguese became an act of heroism to independent Brazilians.
Libertas quæ sera tamen means "Liberty, even if late [or delayed]." It is a verse from a Latin poem that was popular then [Virgil's Eclogues--ed.].
From the state legislative assembly's website, the following is quoted from Law No. 2793 of 8 January 1963 (published and took effect on 9 January 1963): Art. 2. The flag of the State of Minas Gerais has the following design and form: a white rectangle 20 units long and 14 units wide; on the center, a red equilateral triangle 8 units on each side, having along the upper left side the word "LIBERTAS", along the upper right side the words "QUÆ SERA" and along the base the word "TAMEN", said words to be in Roman type, with the letters to be 2/3 of a unit high and separated from the triangle by 1/3 of a unit, forming together the phrase Libertas quæ sera tamen, which is the motto of the Inconfidência Mineira.Clóvis Ribeiro in 1933 wrote that the triangle on the flag designed by Tiradentes for the Inconfidência was green, yet 30 years later the state adopted the same flag shown in Ribeiro but with a red triangle, ostensibly after careful research and much discussion. Ribeiro says that at least one unit of Minas Gerais troops involved in the 1930 revolution that brought Getúlio Vargas to power carried a flag with a red triangle and the Libertas quæ sera tamen motto; presumably it was not alone. But I don't know how such flags came into use or whether they were a source for the official flag adopted in 1963.
A set of cards sold with bars of Eucalol soap in the 1930s shows Minas Geraes (as it was then spelled) with the present state flag, which is something of a surprise since the published sources give 1962 or 1963 as the adoption date of this flag. Clearly it was in use no later than the 1930s.