Blizzard have clarified some points regarding the Sparks of Shadowflame in patch 10.1, from their timing to where extra sparks will drop for when the catchup system starts.
Sparks of Shadowflame (Source)
Clarifying a couple of points here.
Sparks are every 2 weeks starting tomorrow (half a spark per week), and there is a tutorial quest that grants an additional half spark. This means players can get their first full spark the same week as the release of the patch, and the second spark will be creatable on week 3 (the week after season 2 starts), etc.
For any characters that fall behind on Spark creation, extra Sparks have a higher chance to drop from sources like:
M+ end-of-dungeon chests
The new World Boss
Your first time defeating the dragon boss of the Fyrakk Assault
The public event Researchers Under Fire
Niffen Cave Dives
Aberrus raid bosses
until the character is caught up.