在英语中，月份前面通常使用介词 in。例如：
in January
in February
in March
in April
in May
in June
in July
in August
in September
in October
in November
in December
当需要表示具体的某一天或某一天的上午、下午或晚上时，应使用介词 on。例如：
on Monday
on Tuesday morning
on the 15th of June
总结：
月份前用介词 in。
具体日期前用介词 on。