1. There's always a lot of traffic at this time of day.

Question

2. There's always a lot of traffic at this time of a day.

3. There's always a lot of traffic at this time of the day.

4. There's always a lot of traffic at this time of days.

5. There's always a lot of traffic at this time of these days.

(Which ones are grammatical?)

Answer 1

1 and 3 are grammatical.