Strong vs weak Day Master measures how firmly your own energy (the Day Master) is supported within the chart. Strong or weak is not the same as good or bad — what matters is how balance is found.
What strong and weak mean
A strong Day Master is one whose energy is firmly supported within the chart; a weak Day Master is relatively less supported. Resource (which helps you) and Companions (which are like you) make it stronger; Output, Wealth and Officer (which drain you) make it weaker.
In practice it is read as a spectrum from extremely weak to extremely strong. Rather than a hard split into strong or weak, it is more accurate to read how far, and in which direction, the chart leans.
Why strength matters
Strength is the criterion that decides whether "the same energy is medicine or poison for me." A strong Day Master welcomes the draining Output, Wealth and Officer; a weak Day Master needs the filling Resource and Companions.
So judging strength accurately is what lets you correctly find the key balancing element (the Useful God). Strength is the single most important step before choosing the Useful God.
Frequently asked
No. Strong and weak are not a ranking but a difference in temperament. Strong is proactive and driven but can be stubborn; weak is flexible and cooperative but can be easily swayed. Each simply needs different support.
You weigh several factors together — whether the birth-month season helps you, whether helping Resource/Companions or draining Output/Wealth/Officer dominate, and whether you have roots (Rooting). It is a process of balancing many elements.