I assume that short term(within a week) returns (completed surveys) results in higher number of returns for the long term. Since Americans have some kind of average days worked and days off with workweek and weekends, I would think there are days of the week when people are more likely to take time (days with spare time?) for a survey. In your experience have you found or tracked days of the American calendar week that get more responses?
I have heard that Tuesday – Thursday is a good window for sending out surveys.
Sending surveys on Mondays can results in the email being buried in the recipient’s inbox (among emails that may have come in over the weekend, etc.). Similarly, sending a survey on a Friday may be difficult in terms of grabbing recipients’ attention as they try to clear out their work email before heading out for the weekend.
My recommendation is to send in the middle of the day on a Tue, Wed, or Thu – but of course this depends on your audience, too. Are you sending to professionals at their place of work? Is it more of a market research survey for consumers who may be taking the survey at home?
A google search for “when is the best time to send an email survey” appears to return quite a bit of information/research about the topic.
Perhaps other users will chime in on this thread, hope this helps!