The "Brexit" vote on Thursday and the release of the results Friday are holding markets hostage with any new poll data sending equities in either direction. 

"Friday is a big day," said Jim Cramer, portfolio manager of the Action Alerts PLUS Charitable Trustin an interview at the New York Stock Exchange on Monday. Cramer said he'll be on the desk at the end of the week to react in real-time to any news coming out of Europe. 

U.S. stocks rocketed higher on Monday as new "Brexit" polls suggested the U.K. could remain in the European Union, bringing a wave of relief (however temporary) after weeks of worry. A Survation/The Mail survey showed 45% of voters wish to "remain" while 42% prefer to "exit." Markets are expected to remain choppy until the outcome of the referendum is announced.

European markets surged on the increasing likelihood of a "remain" vote. Germany's DAX jumped more than 3%, the CAC 40 in France rocketed 3.4% higher, and the FTSE 100 in London spiked 2.9%.

Cramer cautioned U.S. investors to stay their hand and wait-and-see how things shake out after Thursday's referendum. 

"Europe is really rebounding [but] we should not rebound as much in part because we never went down as much," he added on Monday. "That's why I've been saying, please don't come in all the way up here. Wait till things cools off." 

The chances of an exit from the EU have sent global markets, Wall Street included, into a spin, triggering fears over economic and political stability in the region.

"Potential 'Brexit' brings a litany of risks along with it, including currency, trade, and economic growth," said Jason Pride, Glenmede director of investment strategy in a note. "Headline risks associated with an exit include the possibility of U.K. bond ratings being affected and the uncertainty surrounding the British pound. Furthermore, many have noted that the U.K. would need to renegotiate its trade agreements which could have adverse effects on U.K. economic growth."

Campaigning on either side was put on pause on Friday in honor of Jo Cox, a British Labor Party politician, who was assassinated on Thursday. The suspect reportedly had far-right leanings and attacked Cox for her anti-exit stance on the referendum.