Actually Lag Ba-Omer might have been a sad day
In the year 363 Jullian the Apostate emperor of Rome had promised to allow rebuilding of the Bais Hamingdosh. Jullian was the last non-christian emperor of Rome and tried to return Rome to Pre-Christian days.
On Lag Baomer in 363 there was a massive Earthquake in the Gallile which destroyed what was left in Israel and shortly afterwards Jullian the Apostate was killed in Battle and all plans for rebuilding the Bais Hamigdosh were shelved