The total capacitance seen from the crystal looking into the rest of the circuit is called the "load capacitance". When a manufacturer makes a "parallel" crystal, a technician uses a Pierce oscillator with a particular load capacitance (often 18 or 20 pF) while trimming the crystal to oscillate at exactly the frequency written on its package.
To get the same frequency performance, one must then make sure that the capacitances in the circuit match this value specified in the crystal's data sheet. Load capacitance CL can be calculated from the series combination of C1 and C2