Write a question about the following article: On December 23, 1918, Milutinović's troops ended a smaller uprising in Rijeka Crnojevića and stopped an attack by the Greens on Nikšić. The next day, about 250 Serbian troops and 850 volunteers from nearby Montenegrin clans fought a formation of approximately 1,500 to 2,000 rebel Greens in Cetinje. On January 6, 1919, the Greens initiated a siege on Cetinje, killing some members of the Great National Assembly and killing some Whites. After that, the Greens experienced severe factionalism, in addition to facing the militarily stronger Whites.

Question about the article:
What day did Serbian troops fight the rebel Greens in Cetinje?