This was the best thing to happen to me ever since self-quarantine happened.
"外出自粛生活が続いている中で最も嬉しい出来事でした"
は英語で This was the best thing to happen to me ever since self-quarantine happened. といえます。
Other ways:
Ever since quarantine happened, this was the best thing that has happened to me!
quarantine = 検疫
This was the best thing to happen so far during quarantine.
".あなたのカードを受け取ったことは、外出自粛生活が続いている中で最も嬉しい出来事でした"
Getting your card was the best thing to happen to me during this (quarantine or stay at home period).
アメリカ合衆国 
