"We are in autumn."
Translation:Noi siamo in autunno.
Hm that's used when referring to a season in general, as in "d'estate si va al mare" or "d'inverno vado in montagna": rather than speaking about a specific summer or winter it refers to the average summer or winter. Although you could use "in" with the same meaning as well :P
About that, Giuseppe Ungaretti (who is famous among highschoolers for having written during WW1 probably the shortest Italian poems ever) wrote "Soldati": Si sta come / d'autunno / sugli alberi / le foglie (Soldiers: People stay as / in autumn / on the trees / the leaves). A deep reflection on the frail human condition sharpened by war, that I only thought I'd mention because I was reminded of it when thinking of an example of "di" :P