Glen Shinn, Matt Baker and Gary Briers seek to answer this question in their article Response Patterns: Effect of Day of Receipt of an E-Mailed Survey Instrument on Response Rate, Response Time, and Response Quality The premise is that responses to online surveys emailed to respondents differs depending on the day that the survey invitation is sent.

Personally I have always tried to avoid sending out on surveys on Mondays for no other reason than I thought people might be more responsive to surveys once they have shaken off the ‘Monday Blues’. I read with interest then that the authors found no significant difference in the rate of response by day the instrument was e-mailed/received. In addition the authors report no discernible difference in the time to respond or in the quality of responses.

So it would seem that one day is just as good as the other (excluding weekends which were not included in the study). Still, I think I will continue to try and avoid Monday mornings just to be on the safe side.