This circuit is line powered flasher, can be used in any application, and Christmas lamp is one of the example.  Look at the circuit’s schematic diagram below:

scr flash light circuit schematic

The control for triggering the SCR is done by  Q1, Q2 and related resistors. The timing depends on R1,R2 & C1. To change flashing frequency don’t modify R1 and R2 values: set C1 value from 100 to 2200µF instead, this because the resistor sis already set for the proper current and voltage point for proper operation. Best performances can be achieved with C1=470 or 1000µF and R4=12K or 10K. A number of 10 to 20 lamp in series for Christmas trees (60mA @ 220V typical for a 20 lamp series-loop) is common, and this low current consumption only need a cheap SCR devices, e.g. C106D1 (400V 3.2A) or TICP106D (400V 2A), and the last, the most recommended is P0102D devices having TO92 case. [Circuit schematic diagram source: ELECTRONICS WORLD “Circuit Ideas”, June 2000 issue, page 458]