diff --git a/ffi/capi/examples/fixeddecimal_tiny/Makefile b/ffi/capi/examples/fixeddecimal_tiny/Makefile
index 2b5a3d8caff..7f05218343c 100644
--- a/ffi/capi/examples/fixeddecimal_tiny/Makefile
+++ b/ffi/capi/examples/fixeddecimal_tiny/Makefile
@@ -17,10 +17,10 @@ $(ALL_HEADERS):
 	cargo build
 
 ../../../../target/x86_64-unknown-linux-gnu/debug/libicu_capi.a: $(ALL_RUST)
-	RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-flto -Cpanic=abort" cargo +nightly-2021-05-20 panic-abort-build --target x86_64-unknown-linux-gnu --no-default-features --features x86tiny
+	RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-flto -Cpanic=abort" cargo +nightly-2021-04-20 panic-abort-build --target x86_64-unknown-linux-gnu --no-default-features --features x86tiny
 
 ../../../../target/x86_64-unknown-linux-gnu/release/libicu_capi.a: $(ALL_RUST)
-	RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-flto -Cpanic=abort" cargo +nightly-2021-05-20 panic-abort-build --target x86_64-unknown-linux-gnu --no-default-features --features x86tiny --features smaller_static --release
+	RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-flto -Cpanic=abort" cargo +nightly-2021-04-20 panic-abort-build --target x86_64-unknown-linux-gnu --no-default-features --features x86tiny --features smaller_static --release
 
 # Naive target: no optimizations, full std
 optim0.elf: ../../../../target/debug/libicu_capi.a $(ALL_HEADERS) test.c
diff --git a/ffi/capi/examples/fixeddecimal_tiny/README.md b/ffi/capi/examples/fixeddecimal_tiny/README.md
index 7a02c7c7587..0379e2cd0bb 100644
--- a/ffi/capi/examples/fixeddecimal_tiny/README.md
+++ b/ffi/capi/examples/fixeddecimal_tiny/README.md
@@ -9,7 +9,7 @@ https://github.com/rust-lang/rust/issues/60059
 You also need to install the correct toolchains:
 
 ```bash
-$ rustup install nightly-2021-05-20
-$ rustup component add --toolchain nightly-2021-05-20 rust-src
-$ rustup target add x86_64-unknown-linux-gnu --toolchain nightly-2021-05-20
+$ rustup install nightly-2021-04-20
+$ rustup component add --toolchain nightly-2021-04-20 rust-src
+$ rustup target add x86_64-unknown-linux-gnu --toolchain nightly-2021-04-20
 ```
diff --git a/ffi/cpp/examples/fixeddecimal_wasm/Makefile b/ffi/cpp/examples/fixeddecimal_wasm/Makefile
index a98775250c4..ae9099b9176 100644
--- a/ffi/cpp/examples/fixeddecimal_wasm/Makefile
+++ b/ffi/cpp/examples/fixeddecimal_wasm/Makefile
@@ -19,7 +19,7 @@ a.out: ../../../../target/debug/libicu_capi.a $(ALL_HEADERS) test.cpp
 	g++ -std=c++17 test.cpp ../../../../target/debug/libicu_capi.a -ldl -lpthread -lm -g
 
 ../../../../target/wasm32-unknown-emscripten/release/libicu_capi.a: $(ALL_RUST)
-	RUSTFLAGS="-Cpanic=abort" cargo +nightly-2021-05-20 build --release -p icu_capi --target wasm32-unknown-emscripten -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort
+	RUSTFLAGS="-Cpanic=abort" cargo +nightly-2021-04-20 build --release -p icu_capi --target wasm32-unknown-emscripten -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort
 
 web-version.html: ../../../../target/wasm32-unknown-emscripten/release/libicu_capi.a $(ALL_HEADERS) test.cpp
 	emcc -std=c++17 test.cpp ../../../../target/wasm32-unknown-emscripten/release/libicu_capi.a -ldl -lpthread -lm -g  -o web-version.html --bind --emrun -sENVIRONMENT=web -sWASM=1 -sEXPORT_ES6=1 -sMODULARIZE=1
diff --git a/tools/scripts/ffi.toml b/tools/scripts/ffi.toml
index 74a60fb3592..0f306d60339 100644
--- a/tools/scripts/ffi.toml
+++ b/tools/scripts/ffi.toml
@@ -128,7 +128,7 @@ exec --fail-on-error diplomat-tool cpp ../cpp/include --docs ../cpp/docs/source
 [tasks.build-wearos-ffi]
 description = "Build ICU4X CAPI for Cortex"
 category = "ICU4X FFI"
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 env = { "RUSTFLAGS" = "-Ctarget-cpu=cortex-m33" }
 command = "cargo"
 args = ["build", "--package", "icu_capi",
@@ -140,7 +140,7 @@ args = ["build", "--package", "icu_capi",
 [tasks.test-nostd]
 description = "Ensure ICU4X core builds on no-std"
 category = "ICU4X FFI"
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 command = "cargo"
 args = ["build", "--package", "icu", "--target", "thumbv7m-none-eabi"]
 
diff --git a/tools/scripts/valgrind.toml b/tools/scripts/valgrind.toml
index 57c40871978..09a5585e945 100644
--- a/tools/scripts/valgrind.toml
+++ b/tools/scripts/valgrind.toml
@@ -8,7 +8,7 @@
 description = "Pre-build artifacts for use with the Valgrind task"
 category = "ICU4X Valgrind"
 command = "cargo"
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 # Add features here and below if a desired example is not being built
 args = [
     "build", "--examples",
@@ -34,7 +34,7 @@ mkdir benchmarks
 mkdir benchmarks/valgrind
 
 # Re-run the build command only to generate the JSON output (--message-format=json)
-output = exec cargo +nightly-2021-05-20 build --examples --message-format=json --features icu_benchmark_macros/rust_global_allocator --features zerovec/serde --profile bench -Z unstable-options
+output = exec cargo +nightly-2021-04-20 build --examples --message-format=json --features icu_benchmark_macros/rust_global_allocator --features zerovec/serde --profile bench -Z unstable-options
 if ${output.code}
     trigger_error "Build failed! To debug, build examples with `--features icu_benchmark_macros/rust_global_allocator`"
 end
diff --git a/tools/scripts/wasm.toml b/tools/scripts/wasm.toml
index 951210e474d..1a0dac030af 100644
--- a/tools/scripts/wasm.toml
+++ b/tools/scripts/wasm.toml
@@ -8,7 +8,7 @@
 description = "Build WASM FFI into the target directory (dev mode)"
 category = "ICU4X WASM"
 install_crate = { rustup_component_name = "rust-src" }
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 command = "cargo"
 args = ["wasm-build-dev", "--package", "icu_capi", "--example", "icu_capi_cdylib"]
 
@@ -16,7 +16,7 @@ args = ["wasm-build-dev", "--package", "icu_capi", "--example", "icu_capi_cdylib
 description = "Build WASM FFI into the target directory (release mode)"
 category = "ICU4X WASM"
 install_crate = { rustup_component_name = "rust-src" }
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 # We don't care about panics in release mode because most incorrect inputs are handled by result types.
 env = { "RUSTFLAGS" = "-C panic=abort -C opt-level=s" }
 command = "cargo"
@@ -26,7 +26,7 @@ args = ["wasm-build-release", "--package", "icu_capi", "--example", "icu_capi_cd
 description = "Build WASM examples into the target directory"
 category = "ICU4X WASM"
 install_crate = { rustup_component_name = "rust-src" }
-toolchain = "nightly-2021-05-20"
+toolchain = "nightly-2021-04-20"
 # We don't care about panics in release mode because most incorrect inputs are handled by result types.
 env = { "RUSTFLAGS" = "-C panic=abort -C opt-level=s" }
 command = "cargo"
@@ -77,7 +77,7 @@ exit_on_error true
 
 # Re-run the build command only to generate the JSON output (--message-format=json)
 set_env RUSTFLAGS "-C panic=abort -C opt-level=s"
-output = exec cargo +nightly-2021-05-20 wasm-build-release --message-format=json --examples --workspace --features zerovec/serde --exclude icu_datagen
+output = exec cargo +nightly-2021-04-20 wasm-build-release --message-format=json --examples --workspace --features zerovec/serde --exclude icu_datagen
 if ${output.code}
     echo ${output.stderr}
     trigger_error "Build failed! See output above."