If you wear makeup it is best to use a cleanser in the morning, and then a cleanser and a scrub (Only every two or three days) at night to fully clear out your skin.
If you do not wear makeup, then the time that you use your scrub doesnt really matter. Most people prefer to use scrubs at night though because sometimes they cause redness which will dissapear by the morning. Just remember only use a scrub 2 or 3 times a week because using a scrub too often will dry out your skin, cause the body to make more oil, which will just give you lots of breakouts.
It is alright to use morning facial scrubs at night time usually, however you should check the ingredients list. Many 'morning' scrubs contain caffeine or guarana which will wake you up. If your scrub has similar stimulants in it, it is best to use in the morning as it may affect your sleeping or make it hard to sleep. Also, some facial scrubs that are labelled 'morning' have a minty or citris scent which wakes you up, so they are better for morning use. However the majority or scrubs labelled as morning facial scrubs are just normal scrubs.
My answer is quite long but I hope it helps you to understand.
I know this because my Aunt is a dermatologist, and she knows lots about skincare and marketing.