You can shock-treat throughout the year. There is no real “best time” to shock-treat.

However, if the water level in your septic tank is very high due to saturated soils (heavy rains, flooded land), it would be better to wait for the soils to dry out a bit before doing the treatment and to avoid using water in the house to give the water level in the septic tank a chance to drop. You can then carry out the treatment without any problems.

It is not necessary to wait until spring to do the shock treatment. You can do it even in winter. The cold of winter does not really affect the system. It is very rare for septic systems to freeze. The biodegradation process produces heat that keeps the system at an adequate temperature even during our harsh Canadian winters.