不用加介詞on .是你查的那樣,英語語法里確實(shí)是這樣規(guī)定的：
英語中作時(shí)間狀語的短語前通常要加in,on,at,for等介詞.例如：
I get up at six o’clock in the morning.我早上六點(diǎn)鐘起床.
但在下列情況下,時(shí)間狀語前不用介詞：
一、當(dāng)時(shí)間狀語是today,tonight,yesterday,yesterday morning/ afternoon/ evening,tomorrow,tomorrow morning/ afternoon/evening等時(shí).例如：
There was a play yesterday afternoon.昨天下午有一場戲.
She will be back tomorrow morning.她明天上午回來.
二、由this,that ,these等構(gòu)成的短語用作時(shí)間狀語時(shí),其前不用介詞.例如：
This term we are going to have a new subject.這學(xué)期我們將有一門新的課程.
He didn’t go home that night.那天晚上他沒回家.
They are working on the farm these days.這些天他們在農(nóng)場工作.
三、由形容詞last,next等修飾的名詞詞組用作時(shí)間狀語時(shí),其前不用介詞.例如：
I saw him in the street sometime last week.上星期我在街上看見過他.
Please come here earlier next time下次請?jiān)琰c(diǎn)來這兒.
四、由every,each等構(gòu)成的短語用作時(shí)間狀語時(shí),其前不用介詞.例如：
Do you do morning exercises every day?你們每天做早操嗎?
They go to school by bike each day.他們每天騎自行車去上學(xué).
五、在以“the day(week,month,year)+before/ after…”構(gòu)成的時(shí)間狀語前不用介詞.例如：
Where did you go the day before yesterday?前天你去哪了?
They came to this school the year before last.他們是前年來到這所學(xué)校的.
六、在以all 構(gòu)成的短語作時(shí)間狀語時(shí),如：all day,all the week,all the year 等前面通常不用介詞.例如：
He was busy all day yesterday.昨天他忙了一整天.
She was ill in bed all the week.她有病了,臥床整整一個(gè)星期.
All that afternoon,I jumped and sang and did all kinds of things.那天整個(gè)下午,我又唱又跳,各種各樣的事都做了.