At, On, In : Grammatical explanations will be unnecessary in this case.  The examples given will clarify the usage.

AT :

• He arrived at 10 o’ clock.
• I will meet you at Bolgatty Palace.
• At present I have no idea to marry her.
• He is very busy at the moment.  I am afraid you can’t meet him now.
• At midnight, at sunset, at lunchtime etc

Note :  We don’t usually say, ‘ At what time…?

• What time are you going out this evening?

ON :

• She arrived on Friday.
• He will fly to New York on 25th December.
• On Christmas day
• On my birthday

IN :

• In the morning/ afternoon/ evening
• In December, in 1999 etc

Note:  We do not use  AT/ON/IN before Last/ Next/ This/ Every.

• I will see you next Friday.
• They got married last February.
• I am meeting him this evening.
• We met him last month at Hotel Indraprastha.