Well the following site create a whitepaper on the subject:

whenisgood.net/static/press/WhenIsGood-Whitepaper-Oct09.pdf

To sum it up from the paper:

Event invitations are most likely to be accepted for a Tuesday at 3pm.
• Very little variation exists between the days of the week. We found no evidence that people tend to be any more flexible in the middle, cost
or at either end, advice
of the week.
• While individual respondents – or particular events – may feature much greater or lesser degrees of flexibility, viagra
on average only three or four people out of ten will be available at any given time.
• Unsurprisingly, the worst hour to suggest is right at the start of the day.
• Flexibility jumps up at 10 and 11 in the morning.
• At lunch time flexibility drops back down, but not by a significant degree.
• The afternoon appears to have most availability, with the 3PM peak falling off rapidly towards the end of the day.

I highly recommend their services:   whenisgood.net