Sorry I do not no understand what you are trying to achieve, please provide more information. If possible give an example or explain your use case. Once we have more information we will be in a better position to assist.
We would like to learn more about yourself, what you do, your interests, projects you are working … please introduce yourself. Whilst you are it, please add a profile picture.
Do you need to use the day name during data collection in the field? Or you require that at data analysis? If its at data analysis I suggest you do the conversation post data collection.
You can use the format-date function as described here. Please note that in Collect v1.9.1 and previous the day and month names ( %b and %a) are always shown in English rather than being localized to match Collect's language. The next release should fix this.