Blood test dates advice

My doctor has told me to go for a blood test 7 days before AF is due to check progesterone levels. I use 2 different apps to track ovulation etc (glow and fertility friend). FF says AF is due on the 17th, meaning I should go on Friday 10th, however glow predicts the 19th, which is a Sunday so should be tested on Monday 13th. Because of the weekend this is a big gap, which date should I go on? I only have one referral letter so can't do both... Thanks in advance for any advice!