How can I choose the Fiesta mode? In Team arena I only get random modes like showdown or ctf
You can´t. It was only available last week till friday. You can now play team snipers for one week and than another playlist will come.
Only a weekend playlist. Sorry.
Temporary playlists are a -Yoink-.
Ok thank you guys, this is sad, i think it’s a very nice mode
Is it possible to choose a mode in the arena or is it only possible to play in ramdom rotation?
They added it as a weekend playlist so today and tomorrow is your best chance to play a few rounds 
it will keep coming back in the week long rotations…so dont worry 
agreed, fiesta is the best mode imo…had great memories doing fiesta slayer in 4 player lan parties in halo 3/reach