referendums is the preferred pluralisation
As the Latin word has no plural as it is a gerundive, like 'addendum'.

According to one source anyway. The Guardian's house style guide is also on the side of 'referendums'.
(dozers, do fuck off ur a nob m8, Wed 23 Jan 2013, 12:36, 1 reply, 13 years ago)

Both are acceptable.
I was merely making a poor joke because he said people are stupid.
Also I'm the kind of wanker who likes to refer to stadiums as stadia.
( tangledupinblue hairy badge with moving eyes, Wed 23 Jan 2013, 12:38, Reply)

I like to refer to more than one haggis as 'haggises'
When it comes to plural forms I am very much a traditionalist.

And yes, both are acceptable, but one is preferred. You know, like bisexuals in theory like both genders, but they always prefer one over the other. Am I right? Fuck knows, I haven't had sex with guys since the nineties.
(dozers, do fuck off ur a nob m8, Wed 23 Jan 2013, 12:45, Reply)