Any night of the week is as good as any other. But they definitely prefer moonless nights for test flights, when the silhouette of whatever they are flying cannot be seen against the moonlit sky.
Because of the high level of compartmentalization, many of the regular employees at the base are not cleared to see the test articles. Therefore, good times to see a test flight are just after the last regular Janet commuter flight leaves for Vegas around 8.15pm, or just before the first regular flight arrives around 4.45am in the morning. Sometimes test flights are done on weekends, when most of the regular employees are off work.