When is the best time to check BP and how many times a day?

Well, it depends. Give us more information on your situation.

Probably best might be mid morning, mid-afternoon and then mid-evening for a short while. Unless there are drastic swings during the day that seem to follow a pattern, you can then switch to once a day testing.
I choose mid evening when I am comfortable in my recliner in front of the tube.
Make sure that it is several hours or more since your BP meds so you can be sure they are working but not at full throttle.

I sit still for about 10 minutes and take 3 readings a few minutes apart and toss the high and low readings and record the middle values. If they are TOO far apart, I retest an hour later...and record BOTH middle values. Some days I record 3 or 4 values, some days only one (and some days I forget completely :D.)
I bring the values in at every doctor's appointment along with an AVERAGE systolic and diastolic for the period.