According to wookiepedia, Maul takes over Mandalore in early 19 BBY, so that probably means child Din Djarin was rescued around 20 BBY. His rescue must have happened after the Death Watch broke up ties with the Separatists (as depicted in Clone Wars), but likely way before they alligned with Maul.

The CW episode “A friend in need” might be set around the time Din was rescued, since the Death Watch are depicted as openly opposing the Separatists by that point.

This would pose several interesting questions. Did the Mando warrior who took Din with him support Maul, Bo-Katan, or was he minding his own business? Where was Din during the siege of Mandalore?