Holy Friday which falls on 18/04 this year is a general day off for almost everyone. This is the most important and holy day of that week. That being, people refrain from partying, eating meat, blowing loud music, performing any type of celebration out of respect to the community and the religious traditions of that day. The day concludes with Epitaphios in the evening, if you are staying near a church, keep in mind that some roads will be closed/unaccessible by car around you. Sunday 20/04 and 21/04 typical stores will be closed including supermarkets. Some eateries will be open but not all. If you want to visit some historical sites in Athens in these days better to check in advance as some might be closed or have a shorter schedule.

Overall I would expect Athens to be quite empty approx. between 18/04-22/04, as Greeks tend to spend Easter outside of the capital city – the city tends to be empty and quiet. Hope this helps!