A question - when do the timer ISR get activated?

Another thing - you may suffer a bit of jitter when trying to generate a zero-crossing interrupt for both halves of the curve. The zero-crossing detector normally always have a bit of offset error and the more narrow window you configure, the more sensitive you will be for disturbances on the 50Hz signal unless you make sure that your zero-crossing detector is fed by a heavily filtered 50Hz signal.

It is often better to have a 50Hz instead of 100Hz zero-crossing interrupt, i.e. only catch the low-to-high or high-to-low transition. Then let the interrupt handler run a little software filter. This filter then computes the timer value when you should turn on the power. If using a triac, it will automatically turn off on the next zero crossing, so you will only need to compute the turn-on times. When turning on the triac for the odd half period, you then know that you would wait a further 10ms to turn on the power for the even half period.