Good afternoon Pau and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the main header becoming huge while zooming on the iPad.
I went ahead and checked your website with my own iPad and it turns out you are absolutely right. That being said, that is true of any page on your site, not just the calendar ones.
So it looks like this is a question for your WordPress theme author.
With that in mind, I believe what might not be helping is the z-index of the header:
#main-header {
z-index: 99999;
}
With that in mind, could you tell me which version of Divi you are using ?
I am asking because I cannot seem to reproduce the issue on my install.
Best regards,
Geoff B.