Because webinars don't require people to go anywhere, there's a lot of flexibility in deciding when they can be scheduled. Most presenters have not adjusted to this brave new world of freedom or the kind of things they need to think about when scheduling their webinar. Sometimes the best time for a webinar is completely counter to when you think the best time for a face-to-face seminar should be held.
First, and this you would think is the most obvious, is to be sure you arrange your webinar so that it works with the time zone in which your attendees live.
Depending on whether you are looking for a national or international audience, you may need to think, about scheduling your webinar twice.
Believe it or not, industry surveys have found out that if you are targeting professionals, Mondays and Fridays generally have good attendance, not what you would think. But these are bad days if your target is the support staff.
Also summer months and holiday weeks, like the beginning of the week of Thanksgiving or the week between Christmas and New Year can work well, though scheduling can again depend on your target audience. For example if you are looking to attract express delivery people, anytime two months before Christmas is off limits.
Because other meetings usually begin and end on the hour, start your webinar at a quarter after the hour and end on the hour or an hour and a half later. This lets your attendees get back from their previous meeting and get ready and lets them finish your webinar before they have to get to their next meeting.
Try to avoid the lunch hour. The best times to consider are generally 10 A.M. and 2 P.M. because people are already at work, generally no matter how late their workday starts, and your webinar will be over before those who come in early have to go home.