Prepositions of Time At-In-On

The preposition " at "  

CASE -1 time indicated in the sentence is sharp and precise.
Example
1) I have to catch the train at 1 p.m.
2) Church close at noon.
3) I would like to talk to you at dinner time.
4) I love reading at bedtime.
5) The best time to offer prayer is at the sunrise or at sunset.
6) The show starts at 7.30 pm.
7) Its prohibited to go out at midnight.
8) I will meet you at the start of the match
note-'at't is used here with time of the day-sunrise,sunset,noon,midnight
CASE-2  at holidays or festivals
Example
1) I will go to home at Diwali
2) Maria is too busy at Christmas
CASE -3 at an age
Example
1) I would going to buy a car at an age of 18.
2) At 16, he joined his father's business.
CASE-4
at night
at the weekend
at the same time
at present
at any moment

The preposition "in"

used in a period(period of time/months/seasons/years/centuries)

Example

1)in 2 hrs.

2)in 4 days.

3)in February.

4) in summer

5)in 2011

6)in 20th century.
7)in the morning
8)in the afternoon

The preposition "on" 

used for days/dates/special days

Example

1) on Monday.

2) on 29 August..

3) on Father's day.

4) on my birthday.

5) on Independence day eve.
6) on Monday morning