|3 people marked this as a favorite.|
The summoned creature gets to act immediately. From page 195:
SummonedA creature called by way of a conjuration spell or effect
gains the summoned trait. A summoned creature can’t
summon other creatures, create things of value, or cast
spells that require an expensive material component
or special focus. It can take only 2 actions on its turn,
and can’t take reactions. Otherwise, it uses the standard
abilities for a creature of its kind.
When you finish casting the spell and when you spend
an action to Concentrate on the Spell, the summoned
creature then takes its 2 actions. After its actions, you
continue with the rest of your turn. You can direct a given
summoned creature only once per turn; Concentrating on
a Spell for a summoned monster more than once on the
same turn doesn’t give that monster any more actions. If
you don’t Concentrate on the Spell during your turn, the
creature takes no actions, assuming it isn’t dismissed due
to the spell having a duration of concentration.
Summoned creatures can be banished by various spells
and effects and are automatically banished if reduced to
0 Hit Points, or if the spell that calls them is dismissed.
You could argue that the text in Summon Monster overrides the general rule, but because it's parenthetical, it seems more likely to be an incomplete reminder of how the rules work.