Yes, you can book your seats on April 5, 7, 8 when you arrive Japan, although I think you don't have to: I mean you can easily book your seats even in the high Cherry Blossom Season.
BTW, where are you going on April 5-7?? I think you should stay for at least several days in Kyoto in the first week of April, so you should be in Kyoto at least on April 5-7. Alternatively, I recommend you to move to Kyoto earlier on April 3 and stay there until April 5. This is because no other place have a variety of Cherry Blossoms including Early-, Standard-, and Late-Blooming Blossoms as Kyoto, as I wrote here.
Indeed, Standard-Blossom which usually bloom in early April bloomed and was done in late March this year in Kyoto, but people who visited Kyoto in early April could still enjoyed the Late-Blossom in Kyoto in early April, as follows.
Therefore, if you visit Kyoto in the first week of April, you can enjoy either Standard or Late-Blossoms.
All you need to do when you arrive Kyoto is to visit "Kyoto Tourist Information Center" at first, on the 2nd Floor of the huge Kyoto Station Building Complex, next to Isetan Department Store, where you can learn from its English-speaking staff, which Temples, Shrines, and Gardens are the best to see Cherry Blossom NOW, as follow!!