I don't know if there are days with runway repairs or something, but the first week of April will be risky.
That is to say, the financial year in Japan ends in March and starts in April, and the first week is in the spring holidays.
So if you visit an airbase on the first week, there is a high chance that you'll see an empty airbase.
The second or third week will be less riskier, because the new financial year will have started and planes will start flying again.
Hope it helps,