Why is the ninth month called September?
early 13c., triws, variant of trewes, originally plural of trewe "faith, assurance of faith, covenant, treaty," from Old English treow "faith, treaty," from Proto-Germanic *trewwo (cf. Old Frisian triuwe, Middle Dutch trouwe, Dutch trouw, Old High German triuwa, German treue, Gothic triggwa "faith, faithfulness"). Related to Old English treowe "faithful" (see true).
The Germanic word was borrowed into Late Latin as tregua, hence French trève, Italian tregua. Trucial States, the pre-1971 name of the United Arab Emirates, is attested from 1891, in reference to the 1835 maritime truce between Britain and the Arab sheiks of Oman.