Well, that depends on what you're translating. Without further context, there is some ambiguity in your sentences. 'These days' can mean 'nowadays', as opposed to 'in the past' or it means 'on these particular days' or 'these particular days'. I suspect your starting with 这些天 or 那些天， in which case I would go for 'On these days'. Without wishing to be unkind, I think your sentences sound somewhat Chinglish, so I will make a couple of suggestions.
(1) These days, our school has a lot of exams, so I have to study hard.
In the past we didn't have many exams. Nowadays, we have a lot of exams, so I have to study hard all the time.
24th, 26th and 27th. On these (particular) days, a lot of exams will take place, so I need to prepare well.
(2) These days, there are a lot of exams, so I have to study hard.
In the past we had few exams. Nowadays, we have a lot of exams, so I need to study hard all the time.
24th, 26th and 27th. On these (particular) days there are a lot of exams, so I need to prepare well.
