on these days/ in these days?

"On these days", "in these days", and "these days" are all grammatically correct, but carry different meaning.(1) these days: = nowadays, at the present time, now 當今，今時今日，現在這些日子
e.g.:
People are more health conscious these days. (在這句，不可以說或寫"in these days", 或 "on these days")

(2) in these days: = during these days 在這段曰子裏
e.g.
August 1st to August 15 this year was the warmest period of the year. In these days the sale of air-conditioners was substantially higher.

(3) on these days = on these (particular) days (在這些曰子裏 (不一定是連續的)
e.g. We usually eat at my parent's every sunday. On these days we would eat more than the rest of the week added together.

Regarding tenses, for "these days", use present tense, because it refers to the present time. For "in these days" or "on these days" use whatever tense that is appropriate for the "time" referring to. For example, in (3) above, if a "past" time is referred to, then the past tense will need to be used:
"We ate at my parent's every sunday for the past 2 months. On these days we ate more than the rest of the week added together."

In These Days

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好似in these days順口啲 ^^"

these days有最近既意思, 所以用present or present prefect會比較適合啲。

on these days/ in these days?

Both are used in oral English.

Both mean these days - for examples

I have been very busy lately / recently/these days.

no need to add in or on but people do not count your grammar during conversation so it does not matter.

However, we better speak or write proper English whenever we could