Usually when a new employee joins the organization he/she will be credited with Sick and Casual leaves on pro-rata basis whereas the earned leaves are credited after one year in service or some of the companies follow Jan- Dec calendar in which case the proportionate leaves will be credited to start afresh from next Jan onwards.
Though Sick and earned leaves may be allowed to be combined when one runs out of sick leaves, casual leaves are basically for attending to urgent personal works on a working day. They are not supposed to be combined with any other leave[company policy should dictate these].
Most of the companies do not allow more than 2 CLs on a trot and if it exceeds it will be treated as earned leaves [company policy matters].
Regarding taking of leave on Friday and Monday, usually this can be availed if it is earned leave. For sick leave of course one has to produce the doctor certificate whereas CL are not usually given over a spread of four days on a trot. Calculation of leaves if taken on Friday and the subsequent Monday will be treated as 2 if the company is following the working days in a month as those on which the employees actually work [excluding weekends / holidays].
Leaves are credited only once a year in general.
Earned leaves are usually 15 per year though the company may give more than 15.
Thanks and Regards