Numbers 10:9 - "And if you go to war in your land against the enemy that oppresses you, then you shall blow an alarm [rua] with the trumpets; and you shall be remembered before YHWH your Elohim, and you shall be saved from your enemies."
Numbers 10:10 - "Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, you shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your Elohim: I am YHWH your Elohim."
The word "alarm" in verses 5 & 6 is "teruah" and the word "alarm" in verse 9 is "rua," #7321, which is a root of "teruah" and means, "to mar (especially by breaking) fig. to split the ears (with sound), ie. shout (for alarm or joy)."
By these verses we can see that "teruah" is associated with alerting Israel of impending danger or war. It is an alarm sounded by blowing the two silver trumpets.
Notice, in verse 9, that when the trumpets are blown, YHWH will "remember" Israel and save them from their enemies. I believe this is why the word "memorial" is used in Leviticus 23:24. Unlike the Sabbath or Passover which are memorials of creation and the exodus, the Day of Trumpets is not a memorial of some past event for us to remember. It is a memorial for YHWH to remember His people when the alarm is sounded. Verse10 suggests the same idea concerning the trumpets being a memorial for YHWH.
