Hi!

I am not sure..event cards can be played during Planning Phase (except the cards that have the text written to be played in this phase) ?

Yes

I was under the impression after reading the learn to play manual, that only allies and attachments can be played during planning phase.

Allies, side-quests, and attachments can only be played during the planning phase (with some exceptions through Tactics Imrahil and Hirgon). Event cards can be played whenever there is an action window, including during the planning phase (unless otherwise indicated on the card, for example Captain's Wisdom can only be played during the Resource phase).
This means that you are allowed to play card draw events during the planning phase, and then play any allies or attachments that you drew during the planning phase (assuming you can pay for them).

" Allies and attachments can only be played in the planning phase" is true, but "Only allies and attachments can be played in the planning phase" is not true. Got it?