We all would like higher survey response rates, and researchers have studied meta-data to try and determine the best day of the week and the best time of the day to send out your survey email invitations.
And the most succinct answer is... It Depends! At least concerning what day of the week you send out your web survey email invites. It depends if you are sending a business to business survey (B2B) or business to consumer questionnaire (B2C). Research shows that for a B2B surveys or white paper the middle of the week is most effective. This means Tuesday through Thursday. A Friday B2B email can get overlooked, and if missed, will become buried in the pile of emails come Monday morning, effectively becoming lost in the shuffle.
B2C survey emails are most effective on nights and weekends, because most people work they typically will not use work time to answer a non-work related survey (such as a customer satisfaction survey concerning a leisure product they recently purchased). Research attempting to discover if a certain time of day that survey emails are sent out has shown that time of day is not a critical factor for influencing questionnaire response rates.