During the April Rila Mountain is snow-covered (there is steel winter).
Many huts are closed due to avalanche danger. You can hike without special equipment
(snowshoes, crampons...)in the tree-line.
There are some huts that work at that time at the level below 1500 m.
Camping is possible only with special equipment.
Also there are other mountains as Rhodopes and Balkan that are more appropriate for winter walks.