The best time to prune a plum tree is spring for young trees mid summer for established one.  This allows the tree to recover quickly and avoids bacterial and fungal infections. Though pruning judiciously in spring after bloom insures that next year's flower buds are not removed.  Can help to improve the trees vigor and fruiting. 
Avoid winter pruning due to wet weather issues.  Silver leaf disease etc.
Remove water shoots in early spring before they harden.
Prune for an open center allowing for max air flow, sun exposure and easier access for harvesting.
Remove weak growth, spindly growth, branches that cross over each other and dead or diseased ones.  This can be done even when the tree is fruiting. 
Trim other branches back to 5-6 buds.

Thank you - I am planning on summer pruning my plum but was hoping for guidance on when in the summer might be best.

Thank you,

Marianne

On Mon, Jul 1, 2024 at 5:24 PM Extension Foundation <<personal data hidden>> wrote:

I woukd recommend mid July with judious pruning as I mentioned in my first response.
Then later in August before rains some additional pruning. Careful not to chop off next seasons flower buds.