  H\"aggstr\"om, Peres, and Steif (1997) have introduced a dynamical version of
percolation on a graph $G$. When $G$ is a tree they derived a necessary and
sufficient condition for percolation to exist at some time $t$. In the case
that $G$ is a spherically symmetric tree, H\"aggstr\"om, Peres, and Steif
(1997) derived a necessary and sufficient condition for percolation to exist at
some time $t$ in a given target set $D$. The main result of the present paper
is a necessary and sufficient condition for the existence of percolation, at
some time $t\in D$, in the case that the underlying tree is not necessary
spherically symmetric. This answers a question of Yuval Peres (personal
communication). We present also a formula for the Hausdorff dimension of the
set of exceptional times of percolation.
