"Blood pressure guidelines are not updated at regular intervals. Instead, they are changed when sufficient new evidence suggests the old ones weren't accurate or relevant anymore," says Dr. Paul Conlin, an endocrinologist with Harvard-affiliated VA Boston Healthcare System and Brigham and Women's Hospital. "The goal now with the new guidelines is to help people address high blood pressure — and the problems that may accompany it like heart attack and stroke — much earlier."
Typically, more attention is given to systolic blood pressure (the first number) as a major risk factor for cardiovascular disease for people over 50. In most people, systolic blood pressure rises steadily with age due to the increasing stiffness of large arteries, long-term buildup of plaque and an increased incidence of cardiac and vascular disease.
As for when to check your blood pressure, the most important thing is to do it consistently the same time of the day (ask the doctor which time he prefers and also what time in relation to taking your medication). The following article has a lot of good information for someone just starting to monitor their blood pressure: https://www.drugs.com/cg/how-to-take-a-blood-pressure.html
If you had previously been diagnosed with high blood pressure, the new guidelines don't affect you too much, says Dr. Conlin, as you still need to continue your efforts to lower it through medication, diet, exercise, and weight loss. "However, based on new information in the guidelines, your doctor may propose treating your blood pressure to a lower level," he says.
High blood pressure, also known as hypertension (See blood pressure chart below) is called the “silent killer” for a reason — there are no obvious symptoms but it can result in heart attack, stroke and even death. The good news is there’s a lot you can do to maintain healthy blood pressure or get back to one, often without the need for medications.
Your total blood pressure reading is determined by measuring your systolic and diastolic blood pressures. Systolic blood pressure, the top number, measures the force your heart exerts on the walls of your arteries each time it beats. Diastolic blood pressure, the bottom number, measures the force your heart exerts on the walls of your arteries in between beats.