What is the best approach to fix the date in a weekly seminar/colloquium with the potential speakers by email I am organizing a weekly seminar (at specific day of every week) at our department. We will have 12 talks for the next coming semester. We have a list of n potential people for these talks (suggested by seminar committee). It is clear that not every people on this list can actually deliver a talk (for time restrain or personal reasons), so n should be larger than 12. 

The invitation is done by sending an email to the potential persons, asking if he/she is interested to give a talk. We don't necessarily have a phone number of these people. 

What is the best approach to propose the date of seminars to the interested people by email?

If I list all the available 12 dates to the first person who replies positively, he/she may not quickly select the desired date and asking to answer quickly may be sometimes uncomfortable. If I have his/her phone number I immediately call him/her for fixing the date, but some people don't like to share their personal phone numbers, also office public phone numbers are not always working (once I almost spent the whole working day to call someone at his office but nobody was there). Also while I am waiting for someone to select his date, it is difficult to propose the same dates to another person as both may select the same date.
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?).