Major works at Kings cross for the connection to the Thameslink tunnels- there is a very limited service into Kings cross on Good Friday & I think also all day Saturday.
it looks like FCC only have 1 Kings Lynn service departing every 2 hours from KX on Good Friday - other services run to Moorgate or terminate at Finsbury Park.
(e.g. there is a half hourly Royston -Moorgate via WGC service running on the Friday)
EC appear to have 3 departures in a 40 minute period, repeated every 2 hours.