fix: change spanish month names to lowercase
Fixes #1363!
Spanish speaking people do not capitalize their months or days. 
Spanish days and months should be lowercase
diff --git a/src/locale/es-do.js b/src/locale/es-do.js
index 45002d07b..95abee59f 100644
--- a/src/locale/es-do.js
+++ b/src/locale/es-do.js
@@ -6,7 +6,7 @@ const locale = {
   weekdays: 'domingo_lunes_martes_miércoles_jueves_viernes_sábado'.split('_'),
   weekdaysShort: 'dom._lun._mar._mié._jue._vie._sáb.'.split('_'),
   weekdaysMin: 'do_lu_ma_mi_ju_vi_sá'.split('_'),
-  months: 'Enero_Febrero_Marzo_Abril_Mayo_Junio_Julio_Agosto_Septiembre_Octubre_Noviembre_Diciembre'.split('_'),
+  months: 'enero_febrero_marzo_abril_mayo_junio_julio_agosto_septiembre_octubre_noviembre_diciembre'.split('_'),
   monthsShort: 'ene_feb_mar_abr_may_jun_jul_ago_sep_oct_nov_dic'.split('_'),
   weekStart: 1,
   relativeTime: {
diff --git a/src/locale/es-pr.js b/src/locale/es-pr.js
index f8c20c3a9..7b923a988 100644
--- a/src/locale/es-pr.js
+++ b/src/locale/es-pr.js
@@ -7,7 +7,7 @@ const locale = {
   weekdays: 'domingo_lunes_martes_miércoles_jueves_viernes_sábado'.split('_'),
   weekdaysShort: 'dom._lun._mar._mié._jue._vie._sáb.'.split('_'),
   weekdaysMin: 'do_lu_ma_mi_ju_vi_sá'.split('_'),
-  months: 'Enero_Febrero_Marzo_Abril_Mayo_Junio_Julio_Agosto_Septiembre_Octubre_Noviembre_Diciembre'.split('_'),
+  months: 'enero_febrero_marzo_abril_mayo_junio_julio_agosto_septiembre_octubre_noviembre_diciembre'.split('_'),
   weekStart: 1,
   formats: {
     LT: 'h:mm A',
diff --git a/src/locale/es-us.js b/src/locale/es-us.js
index cc81c4843..9aeecdb43 100644
--- a/src/locale/es-us.js
+++ b/src/locale/es-us.js
@@ -6,7 +6,7 @@ const locale = {
   weekdays: 'domingo_lunes_martes_miércoles_jueves_viernes_sábado'.split('_'),
   weekdaysShort: 'dom._lun._mar._mié._jue._vie._sáb.'.split('_'),
   weekdaysMin: 'do_lu_ma_mi_ju_vi_sá'.split('_'),
-  months: 'Enero_Febrero_Marzo_Abril_Mayo_Junio_Julio_Agosto_Septiembre_Octubre_Noviembre_Diciembre'.split('_'),
+  months: 'enero_febrero_marzo_abril_mayo_junio_julio_agosto_septiembre_octubre_noviembre_diciembre'.split('_'),
   monthsShort: 'ene_feb_mar_abr_may_jun_jul_ago_sep_oct_nov_dic'.split('_'),
   relativeTime: {
     future: 'en %s',
diff --git a/src/locale/es.js b/src/locale/es.js
index 951e7f531..c75588fd4 100644
--- a/src/locale/es.js
+++ b/src/locale/es.js
@@ -7,7 +7,7 @@ const locale = {
   weekdays: 'domingo_lunes_martes_miércoles_jueves_viernes_sábado'.split('_'),
   weekdaysShort: 'dom._lun._mar._mié._jue._vie._sáb.'.split('_'),
   weekdaysMin: 'do_lu_ma_mi_ju_vi_sá'.split('_'),
-  months: 'Enero_Febrero_Marzo_Abril_Mayo_Junio_Julio_Agosto_Septiembre_Octubre_Noviembre_Diciembre'.split('_'),
+  months: 'enero_febrero_marzo_abril_mayo_junio_julio_agosto_septiembre_octubre_noviembre_diciembre'.split('_'),
   weekStart: 1,
   formats: {
     LT: 'H:mm',

diff --git a/test/locale.test.js b/test/locale.test.js
index 7faab1d08..0e6fd06eb 100644
--- a/test/locale.test.js
+++ b/test/locale.test.js
@@ -17,7 +17,7 @@ const NOT_SUPPORTED_LOCALE_STRING = 'not_supported_locale_string'
 it('Uses spanish locale through constructor', () => { // not recommend
   expect(dayjs('2018-4-28', { locale: es })
     .format(format))
-    .toBe('sábado 28, Abril')
+    .toBe('sábado 28, abril')
 })
 
 it('set locale for one instance only', () => {
@@ -27,7 +27,7 @@ it('set locale for one instance only', () => {
 
   expect(dayjs('2018-4-28')
     .locale(es).format(format))
-    .toBe('sábado 28, Abril')
+    .toBe('sábado 28, abril')
 
   expect(dayjs('2018-4-28')
     .format(format))
@@ -40,7 +40,7 @@ it('set global locale', () => {
     .toBe('Saturday 28, April')
   dayjs.locale(es)
   expect(dayjs('2018-4-28').format(format))
-    .toBe('sábado 28, Abril')
+    .toBe('sábado 28, abril')
   dayjs.locale('en')
   expect(dayjs('2018-4-28').format(format))
     .toBe('Saturday 28, April')
@@ -63,10 +63,10 @@ it('immutable instance locale', () => {
   expect(origin.format(format))
     .toBe('Saturday 28, April')
   expect(origin.locale('es').format(format))
-    .toBe('sábado 28, Abril')
+    .toBe('sábado 28, abril')
   const changed = origin.locale('es')
   expect(changed.format(format))
-    .toBe('sábado 28, Abril')
+    .toBe('sábado 28, abril')
   expect(origin.format(format))
     .toBe('Saturday 28, April')
 })
@@ -86,30 +86,30 @@ describe('Instance locale inheritance', () => {
 
   it('Clone', () => {
     expect(esDayjs.clone().format(format))
-      .toBe('sábado 28, Abril')
+      .toBe('sábado 28, abril')
     expect(dayjs(esDayjs).format(format))
-      .toBe('sábado 28, Abril')
+      .toBe('sábado 28, abril')
   })
 
   it('StartOf EndOf', () => {
     expect(esDayjs.startOf('year').format(format))
-      .toBe('lunes 1, Enero')
+      .toBe('lunes 1, enero')
     expect(esDayjs.endOf('day').format(format))
-      .toBe('sábado 28, Abril')
+      .toBe('sábado 28, abril')
   })
 
   it('Set', () => {
     expect(esDayjs.set('year', 2017).format(format))
-      .toBe('viernes 28, Abril')
+      .toBe('viernes 28, abril')
   })
 
   it('Add', () => {
     expect(esDayjs.add(1, 'year').format(format))
-      .toBe('domingo 28, Abril')
+      .toBe('domingo 28, abril')
     expect(esDayjs.add(1, 'month').format(format))
-      .toBe('lunes 28, Mayo')
+      .toBe('lunes 28, mayo')
     expect(esDayjs.add(1, 'minute').format(format))
-      .toBe('sábado 28, Abril')
+      .toBe('sábado 28, abril')
   })
 
   it('dayjs.locale() returns locale name', () => {

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