Amazon extended Prime Day this year to four days instead of two – a change that is expected to rake in big bucks for the online giant.

"It's two Black Friday's which is a huge event," said University of New Haven Economics Professor John Rosen.

Analysts predict sales on Amazon over four days will drive $23.8 billion in overall online spending – 28.4% more than last year. Amazon says it will have deals dropping as often as every five minutes during certain periods.

"Watching carefully to see if the things I want go on sale…I'm actually looking for a pressure washer that I can handle," said Therese DiCenzo from Wilton.

Some may be eligible for 5% cash back on their purchases for a limited time. Amazon says it's offering special discounts on Alexa-enabled products like Echo, Fire TV and Fire tablets.

But analysts warn of fake deal sites and phishing emails.

"Make sure you know who you're dealing with. Make sure you're not giving your password away to anybody you've never met before," said Rosen.

They say to use known secure sites like Amazon and Walmart.

Analysts say concerns about tariffs won't have an effect on goods until next July.

"If the tariffs really do go into effect, they will be inflationary, they will drive prices up," said Rosen.

Best Buy, Target and Walmart are also offering deals this week to compete with Amazon.