Just had a quick look at the crowd calendars on QueueTimes.com for you and it seems the park hasn't opened for Easter weekend before, at least not in the last 8 years which is as far as those calendars go back. Easter's pretty late this year which probably explains why their opening weekend coincides with a public holiday.
On a hunch, I'd expect it to be very busy - my last visit to Liseberg was the final weekend of the summer season (a 'normal' though unseasonably warm September weekend) and it was pretty packed. You can do reasonably well at Liseberg making use of the free virtual queuing service but on busy days those slots are hard to get especially for larger groups. Being in the heart of the city it seems a lot of the locals just flock there to hang out and try to win giant chocolate bars, so the park will probably be at capacity. It's a bit of an unknown with no historic data available, but it feels like a gamble to me.
From what I've heard you might do better to visit in the summer holidays, as a lot of the locals leave the cities and head out to their summer residences in the countryside.