I've recently faced the similar requirement in sharepoint. To send email from workflow @ specific day interval after item has been changed.
you should create custom timer job & use Sharepoint Timer service to send email on regular weekly basis( on every Wednesday)
refer this article by Andrew Connell on how to create custom timer job
In above article SPMinuteSchedule class has been used, for your case you need to use SPDailySchedule to set appropriate day.(eg. Wednesday)
let me know if you face any problem while creating custom timer job.