19 April is a apprehensively approached date of many in the FBI and other similar agencies in the United States because of the history of actions that occurred on that date. Extremist militia organizations often use 19 April as a call to arms date (as did the Militia of Montana in 1995). They point to the following dates as a sign of this pattern:
1775: Lexington burned by British as punishment towards defiance.
1943: Warsaw burned by Germans as punishment towards uprising.
1985: Terrorist enclave takedown of the Covenant, Sword, and Arm of the Lord. This represents the first operation of the FBI's Hostage Rescue Team(HRT). To this day, the HRT takes on a men in black aura because first, the FBI refuses to make a public comment on their operations and second because the team usually wears solid black uniforms void of insignia.
1992: Ruby Ridge operation by HRT. Randy Weaver, an Idahoan separatist had reached a standoff with the FBI, and during the assault, his wife and son were killed.
1993: Waco operation reaches compromised authority status. Branch Davidians ambush and kill/incapacitate several members of HRT after media reveals their planned assault. During the assault, the compound catches fire, and many of the cult members die.
1995: Planned execution date of Richard Snell. Snell was the murderer of a Texas pawnbroker and a policeman. The cause of the two murders is often linked to racism and government paranoia. Also this date, a bomb is exploded in the A.P. Murrah Federal Building in Oklahoma City.
1999: The date of the planned "revenge" at Columbine High School. Apparently, something went unexpected, and the shooters decided to wait a day.
As you can see, it is a popular day for terrorism because of the overtones of a government that has gone too far. In Danny Coulson's book No Heroes, he talks about calling in for a day's vacation, keeping his family at home and sitting by the door with a CAR-15 assault rifle that he had checked out from the armory waiting for someone to make good on their threats. He describes how many people in the HRT received similar threats.
On a personal note: From my time in the intelligence community, his reactions are not uncommon. In fact, one of my best friends birthdays was 19 April, and she never figured out why I always took her to dinner the next day. The paranoia runs deep on both sides, and unfortunately it often leads to rash action.