The warning was quite broad, claiming that some group was planning some attack on some large gathering, including concerts, in Moscow, of which there are many. And it warned Americans to avoid large gatherings for the next 48 hours. That was on March 7th. The actual attack would only take place on March 24th.
Incidentally, there is speculation that the attack may have been planned for March 9th. One of the terrorists was photographed at Crocus on the 7th, and on the 9th there was a large concert by Shaman - a patriotic Russian singer who's regularly made songs glorifying the war in Ukraine, performed for soldiers in Russia's claimed territories, and so on. This would also have coincided with just before the Russian elections, which happened on the 15th. But security was extremely high during that concert - very possibly in response to the US warning.
By contrast when Russia warned the US about the Boston Bomber, the warning was precise to the point of even naming him.
Incidentally, there is speculation that the attack may have been planned for March 9th. One of the terrorists was photographed at Crocus on the 7th, and on the 9th there was a large concert by Shaman - a patriotic Russian singer who's regularly made songs glorifying the war in Ukraine, performed for soldiers in Russia's claimed territories, and so on. This would also have coincided with just before the Russian elections, which happened on the 15th. But security was extremely high during that concert - very possibly in response to the US warning.
By contrast when Russia warned the US about the Boston Bomber, the warning was precise to the point of even naming him.