What is the best approach to fix the date in a weekly seminar/colloquium with the potential speakers by email
Send an invitation to your top 12 candidates. List all 12 dates to all candidates. Ask everyone to reply whether they can and want to come at all, and to indicate their first, second and third choice of dates.

Track answers. Any date that is chosen by exactly one person as his first choice is blocked. Clear any conflicts in first choices among the people involved, using second and third choices.

After one week, remind those that haven't replied yet.
One week later, assume those that haven't replied after the reminder will not reply at all. Assume that k people will come. Send out invitations to your next 12-k candidates, shortening the list of potential dates by those that have already been blocked.
Iterate until all dates have been filled.
Expect people to cancel at short notice. Have someone local as a backup speaker (yourself?).