All the answers you see here are complete and utter tosh.
Because your blood pressures continuously vary from minute to minute and hour to hour every day (and night) the concept of there being a "best time" is absurd, nonsensical rubbish.
If you wish to be well-informed about your readings then there are 2 ways to go about it.:-
(1) using a notepad, keep taking as many random readings as you can, so that gradually you can build up a 'profile' of how your B/P's behave and vary, over the whole 24 hours. Don't stop at one period. Build it up, and you'll find a recognizable pattern emerges, showing how it goes up and down with your Circadian Rhythm, UP with meals, and DOWN with rest and sleep etc. And how just before rising at dawn you get a sudden boost of adrenaline
which makes your B/P's soar temporarily. *(which is why it's simply daft to suggest taking it early morning, because you'll never know if you're doing it before or after the adrenaline rush and/or if it's worn off or not !)
(2) Now you've got your daily profile, you can pick any convenient time, and then simply take readings at that time each day, -or when you want to. Now you can compare how your daily readings (which should be relatively constant for that time of day) actually ARE behaving. An excellent time is late morning, when the Circadian Rhythm usually is at its minimum, the "rush" has worn off, and you haven't eaten any large meal.
It is important to realize that the 'monitor' figures are not "real".... that is, they AREN'T the pressures your heart is actually generating. It's like viewing a cat and a mouse, back-lit by a candle, behind a screen. The mouse may well appear to be bigger than the cat, but it;'s only an illusion).
However, CHANGES in your B/P's ARE REAL, just as the changes in relative positions of the mouse and the cat behind the screen are.
So, if the cat appears to move upwards 10% then it really has (well, nearly!) - or, if the mouse seems to move 10 % sideways, then it actually has, behind the screen.
And so it is with your pressures. If today's readings are up 20% on 'normal for that time of day" then there is a reason. Look for it. Don't suddenly rush off to your doctor and shout "I've got High Blood Pressure
Use common sense, ignore 98% of what you read here in this Zoo, and try to visualize it in the context of the cat and the mouse and the screen.
Of course, there may well be a time when your B/P actually DOES rise for a reason. But it's rare, and then you can safely go to the doctor and tell him -not that it's a high reading, but that it's suddenly CHANGED to a high reading. Now THAT is sensible and meaningful and will be of great help.