What day of the week does my goal start?
The answer to this question is... there really isn't one! Beeminder doesn't care what day it is, or when you do the work, or submit data to your goal — as long as you stay on the good side of the bright red line every day. If you ever end the day on the bad side of your bright red line, you'll derail.
For example, if you start a "6 workouts per week" goal on Monday, the average pace for this goal would be about 0.86 workouts per day: 6 workouts divided by 7 days. If you do one workout per day for the first six days of the week, you'll earn a day off on Sunday. You can do all 6 workouts on Monday and coast the rest of the week! (Not recommended.) But you can't procrastinate and make up a week's worth of workouts on Friday, because you'll have fallen below the average pace before then.
When you create a goal, you can opt to start with some extra days of safety buffer. If you want to "align" your weekly schedule to M-F, you can add extra days so that the first day you're due to work is Monday. That'll give you some extra days to get over an irregular first week schedule — for example, if you started the goal on Thursday. After that, you'll need to stay on track (or better!) on average over the last 7 days, no matter which 7 days those happen to be.