A crowded seed tray can tempt you to move every seedling as soon as green appears. The safer starting point is the guidance for the species and a plant that is developed enough to handle without crushing its delicate stem.
The RHS seed germination guide describes pricking out as true leaves emerge. Its practical module guide also emphasises seedlings being strong enough to handle. Treat the plant’s condition and species guidance as more useful than a universal number of days.
Prepare the destination first
Fill suitable clean containers with the recommended growing medium and make receiving holes before lifting the seedlings. Label them while the varieties are still easy to distinguish. Young plants should not sit exposed while you search for pots.
Moisten the medium appropriately. You want a workable root ball, not a tray flooded so heavily that handling becomes difficult.
Support the roots and protect the stem
Loosen the medium beneath a seedling with a suitable small tool. Handle gently by a leaf as the method permits, supporting the root area rather than pulling on the stem. Move one plant at a time and avoid unnecessarily stripping medium from fine roots.
Set it into the prepared hole at the depth appropriate for that plant, settle the medium gently and water as recommended. Keep the label with the correct group.
Afterward, provide conditions suitable for recovery rather than immediately exposing the seedlings to harsh sun or wind. Watch the plants and adjust care to their needs; pricking out into a larger container is separate from preparing them for outdoor conditions.
A short note of the stage and outcome helps next season. The goal is enough room for continued growth with as little handling stress as practical, not moving the entire tray on an arbitrary calendar date.