Can I take my 20 mg Celexa tablet in the evening before going to bed? If yes, do I need to wait a day to make the switch?

Yes, you can take your Celexa 20 mg tablet in the evening before going to bed. To make the change, the simplest way would be to take half a tablet (10mg) in the morning at your usual time and the other half in the evening before bed. The next day, you could then take the full tablet (20mg) in the evening and continue with that dosage every night. Another option is to gradually delay the timing of your dose by 1-2 hours each day until you reach the desired time. However, this may cause drowsiness throughout the adjustment period, which could take about a week. The first option is likely the most appropriate to avoid continued drowsiness.