Right now I'm using qbo free trial version.I want to buy it in Jan. For not missing the 50% off deal for the first 3 months should i purchase it till end of Dec.2021?

Keep in mind, you have to pay the regular rate when purchasing a trial account. You have to open a new blank QBO account to get the discounted rate.

https://quickbooks.grsm.io/EU

As an additional option, utilize the trial version of a Copy tool to duplicate data from your trial account.

https://rewindio.grsm.io/copy