It's the final trading day of the week and it's going to be busy.

Here are the five things you need to know before the opening bell rings in New York:

1. Markets grumble over Greece: Argh! The Greek drama just never ends. It's really killing the mood.

Stock markets in Europe are falling in early trading, the main Greek index is down by about 4%, and U.S. stock futures are moving lower as investors worry about Greece's future in the eurozone.

Greece needs more money to continue paying its bills, but it's been unable to cut a deal with creditors to give it more cash. The International Monetary Fund pulled its officials from talks in Brussels Thursday, citing a lack of progress.

Traders had been betting Thursday that a deal would be done, sending the Greek market up by 8%.

Related: The 3 sticking points holding back a Greek deal

2. Stock market mover -- Twitter: Shares in Twitter (TWTR) are set to surge at the open after the company said CEO Dick Costolo is stepping down. The stock was up by as much as 10% in extended trading. The company's board of directors named Twitter chairman and co-founder Jack Dorsey interim chief while they search for a new leader.

The term #ThankYouDickC was reportedly trending on Twitter after the announcement.

3. Seal the deal: Tobacco company Reynolds American (RAI) is taking over Lorillard (LO) on Friday, as previously planned. A federal district court just approved the final deal.

The newly combined company, which will sell brands including Newport, Camel and Pall Mall, is projected to have more than $11 billion in annual revenues.

4. Economics: At 8:30 a.m. ET, the U.S. Bureau of Labor Statistics will update the Producer Price Index, which predicts changes in retail prices and inflation.

At 10 a.m., the University of Michigan is updating its Consumer Sentiment Index, which has been declining for most of 2015 as gas prices have climbed.

Related: What are consumers buying? Cars, food and stuff for their home

5. Action in Asia: Most Asian markets ended the week with gains.

Stocks in China had an especially strong session, with the Hang Seng and Shenzhen indexes closing with gains of more than 1%.

The Shenzhen has surged by just over 122% since the start of 2015, making it the best performing stock market of the year.