Time

in + month/year/season → in April, in 2027, in summer 

on + day → on Monday, on Tuesday, on Christmas Day

at + specific time → at 5 PM, at midnight

Differences

The (thuh) - Used before a consonant sound. Eg: the book, the car, the dog 

The (thee) - Used before a vowel sound. Eg: the apple, the end, the idea

Remember it’s based on sound, not spelling.