The Welcome Ceremony is every day, 10 minutes before scheduled park open, weather permitting.
On EMH mornings it is 10 minutes before the EMH park open (so usually at 7:50am for an 8am EMH).
On EMH mornings there is not a second Welcome Ceremony before regular park open. However, I have heard that there is a special recording that plays that includes a reading of the dedication of the Magic Kingdom. I haven't heard it in person, though.