Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
midsummer [mɪdˈsʌmər] n середи́на ле́та
Midsummer's Day день*m ле́тнего солнцестоя́ния
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.