it's highest late morning/early afternoon, BUT, if you're trying to get a good feel for what your blood pressure is doing, it's very important to do two things when taking a measurement:
1) take it exactly the same way every time. that is to say, your body and arm should me in the exact same position each time you take it. this alone can cause great changes in readings if you don't. unless you're consistent with how you do it, you learn and monitor nothing. btw, the best way to take bp is sitting up (but leaning back on something) with the cuff as close to heart-level as possible and your elbow extended (arm pointing away from your body), but your arm should be resting on something (do not hold your arm up or out). and relax for a few minutes in the position you're going to take it before you take it.
2) take it at various times throughout the day, but again, try to be consistent.
hope this helps.