There is currently no direct way to state that a TrainAsONE generated workout should always be scheduled on a specific day / day of the week.
This only becomes a potential issue if you are targeting less runs per week than the number of days that you have allowed TrainAsONE to schedule on. For example, if you are aiming for 5 runs per week and cannot run at weekends, there is no issue. However, if you can only run on week days but targeting 3 runs, there is no mechanism to state that one of those runs is mandated to always be on a Wednesday (for instance).
In this situation, there are a couple of options:
1. Explicitly mark other days that are your least preferable to run to force an exact set of days to run upon. Additionally, if the mandatory training day is generally a significant workout day, you can remove some or all of the constraints after a couple of weeks, as the system will tend to keep to this regular pattern of training.
or 2. Enter the mandatory training day as a specific workout via your Training Settings.