On what date did Tonga's constitution codify its current flag design?

Tonga’s constitution codified its current flag design on 4 November 1875.

The flag has a red field with a white canton at the upper hoist, bearing a red couped Greek cross. Its Christian symbolism reflects the conversion of King George Tupou I and the strong influence of Wesleyan missionaries in nineteenth-century Tonga. Prince Uelingatoni Ngu Tupoumalohi is generally credited as the chief designer, with missionary and statesman Shirley Baker also involved in developing the national symbols.

An earlier Tongan design placed a cross on a white field, but it closely resembled the flag of the International Red Cross, adopted in 1863. The cross was therefore placed in a white canton on a red field. Article 47 of Tonga’s constitution states that the national flag must never be altered, making the 1875 constitutional date more precise than simply saying the flag was designed in that year.