Monday briefing: Last stand for a key city in Ukraine; deadly Airbnb shooting; Tax Day; Boston Marathon; and more

Russia is poised to take over the port city of Mariupol.

• What’s happening: Ukrainian forces defied Russian demands to surrender yesterday, but the situation looks bleak.
• Why Mariupol is a key battleground: The area provides strategically important access to the Black Sea and beyond.
• What else to know: Officials say at least seven people were killed by missile strikes on Lviv this morning, the first wartime deaths in the western city near Poland.

There was a deadly shooting at a party in a Pittsburgh Airbnb.

• The details: Over 90 shots were fired early yesterday, killing two boys under 18 and injuring at least eight other people, according to police. Many of the 200 people there were underage.
• What happened? A fight broke out before the gunshots started, police said, and they believe there were multiple shooters. No one has been arrested.

Crime is rising on public transportation across the country.

• It’s a growing trend: Last week’s shooting on a New York subway train is the latest example, but there has been a string of high-profile incidents this year.
• What’s behind this? Some of it is tied to a wider spike in crime in the U.S., experts say, as well as the fact that many trains and stations are emptier than they used to be.

Your taxes are due today.

• The fine print: If you owe money, the payment needs to go out today(or tomorrow, if you live in Maine or Massachusetts); if not, there’s no penalty to get an extension and file later.
• How to file: If you’re expecting a refund, filing electronically is the fastest way to get it. You can do that through the IRS’s Free File Fillable Forms or services like TurboTax, although those have privacy trade-offs.

Public libraries are becoming the next censorship battleground.

• What’s happening? Conservative activists in several states, including Texas, Montana and Louisiana, have teamed up with politicians to remove books and cut library boards.
• Why? They’re building on a movement that started in schools, going after books that deal with race, sex, gender and other subjects they consider inappropriate.

A bird flu has spread to 27 states, driving up the cost of eggs.