If I receive a mail in the iPhone with the text "tomorrow morning", this text turns into a link for "tomorrow morning" in the calendar app. The same doesn't happens on the iPad. Is there a way to enable it?
How odd. On my iPad I do get links to the Calendar app, for the text 'Tomorrow morning', just as on iPhone.
I just looked in the Settings app, under 'Mail, Contacts, Calendars', for an option to turn this facility off and on. There isn't one.
So I don't think the issue is the iPad itself. I suggest you look carefully at the email itself, e.g. to see if there is any difference in punctuation or spacing, between what works on your iPhone and does not work on the iPad.