It is grammatically correct but does not show the best idiomaticity, in my opinion. Something such as the following would have seemed more usual to me.

• Now they've reached this moment [during which/when/where] they were planning to release a bill last week.

There is no difference as to meaning; "where" can be used to stand for places, situations and stages (Cambridge Dictionary). The particular moment mentioned can be identified to a given situation or stage in a progression. The given form used communicates somewhat different connotations. For instance the same general idea could have been written like so:

• "Now they are the point where they were planning to release a bill last week.".

It is grammatically correct but does not show the best idiomaticity, in my opinion. Something such as the following would have seemed more usual to me.

• Now they've reached this moment [during which/when/where] they were planning to release a bill last week.

There is no difference as to meaning; "where" can be used to stand for places, situations and stages (Cambridge Dictionary). The particular moment mentioned can be identified to a given situation or stage in a progression. The given form used communicates somewhat different connotations. For instance the same general idea could have been written like so:

• "Now they are at the point where they were planning to release a bill last week.".