When a scheduled session is about to start, the email subject is "Your session starting soon" The other emails I get are worded properly, e.g. "Your series ... is published!" "Your series is full!" "24 hours until your ... session!" "Time to give your learners feedback after your ... session!" "A peer tutor has shared feedback with you!" etc. Those all end with exclamation points, so perhaps "Your session is starting soon!" if we're looking for equivalent levels of enthusiasm in these messages.