Is it bettet to take citalopram in the morning or at night?

Morning. Most psychiatrists prescribe citalopram (and other ssri's like paroxetine, fluoxetine, s-citalopram, sertraline) in the morning, because ssri's can cause insomnia in many people. However, in some individuals, ssri's can cause tiredness, fatigue, or sleepiness, in which case you could take the citalopram at bedtime.
Either. Either is fine. If it is causing drowsiness or nausea then take it at night. If it keep you up then morning.
It doesn't matter. Most people take it in the evening because it may make you a little tired. However, it is equally effective in combating depression regardless of the time you take it.