Midsummer or Litha takes place on the summer solstice (around June 21st in the northern hemisphere and December 21st in the southern hemisphere.  This is my explanation of this holiday.  It may or may not work for you and your path.  There are a number of things that this Sabbat usually celebrates: 

•  The power and strength of the sun
• Various sun Gods
• The longest day of the year
• The Oak King takes over for the Holly King

Some ways of celebrating culturally are:

•  Have a bonfire
• Eat foods that honor fire (usually spicy)
• Eat seasonal foods
• Go on a picnic
• Go for a hike
• Drink mead, but drink responsibly
• Have a drum circle
• Dance
• Host a barbeque
• Watch the sun rise
• Spend time with family
• Pick herbs
• Make a flower or oak leaf crown or wreath
• Pick seasonal foods (where I live, this means we go strawberry picking)
• Watch A Midsummer Night’s Dream

Some ritual ideas are:

• Fire magic
• Drum or make other music
• Dance or move
• Honor a sun God
• Reaffirm your relationship to any deities you work with