If you are planning to Set a monster this turn, instead of Summon, you should wait until after your Battle Phase.
That way, if you attack and your opponent has a Flip Effect like "Cyber Jar", your Monster Card will still be in your hand and will not be destroyed. Then you can Set it during Main Phase 2.
If you are planning to attack with your new monster after Summoning it, instead of Setting it, then you usually do it in Main Phase 1. But this can be risky if your opponent has a Trap Card that can destroy your monsters, so sometimes it is still smarter to wait until after your Battle Phase. |