Why can't it be 'is' too? Aren't we talking about identity?
(Looking at the schedule) It seems that every tutorial (yesterday's, today's, tomorrow's etc.) is an interactive session. Yesterday's tutorial is an interactive session.(Identity) Tomorrow's tutorial is an interactive session too.
The interaction occurred in the past for the previous day's session. That is why it can't be is. You are describing the nature of a session that occurred in the past. It isn't still interactive; it was interactive yesterday. The session is over; no one is currently interacting.