Chatted with Nic, he assures us that there's an easy way to do this in the works, but here's an alternative in the meantime:
:: We have a date field called 'Date'
:: Add a field in the builder called 'DayNumber', and make it an 'equation'->type:'date'->date type:days->result type: numbers, and in the formula box just put your 'Date' field
:: This will calculate the number of days that have elapsed since the beginning of computer time (16,000+ days ago, give or take). Don't worry that it doesn't look helpful, nobody except you will see this part
:: Add a field in the builder called 'DayOfWeekNumber', and make it an 'equation'->type:'date'->date type:'days'->result type:'number', and in the formula box put 'DayNumber%7'. This is the modulus (mod), which is essentially the remainder after dividing by 7. So, 9%7=2, since 9 / 7 = 1 Remainder 2, 13%7=6, and 14%7=0
:: It turns out that day 1 is a Thursday, so any record where the 'DayOfWeekNumber' is 0 is a Thursday. For our purposes, we needed to know if the day was a weekend, so we're looking for a 'DayOfWeekNumber' of 2 or 3 to signify a weekend day, otherwise it's a weekday.
:: Now you can use the result of this in a calculation or equation field. If you need to display the day, you can use 'Rules to set field values' by clicking the little gear icon on the right edge of the field view to output the day of the week.
Any questions, post below, I keep a pretty close eye on here.